short answer...Yes you can.
Now the book explanation and suggestion....
the only thing that REALLY matters is that they have the same GPU (9800) the clock speed difference is simply a factory setting. I would however suggest that you either overclock the 9800GTX to match the clock of the 9800GTX+ or underclock the GTX+ down to the GTX speed
The next thing that would play a part is the vram. you can run cards of different sized vram but they will both perform at the level of the smaller ram. such as if you run a 512 and a 1G you would lose half the ram on the second card. having a total of 1G vram vs 1.5G
that said....I WOULD STRONGLY SUGGEST finding out what the total amp output of your PSU (power supply) is. Most Thermaltake 700W are rated at 56A which is pretty good for a 700W. but hter is aleast one model I cannot find the amp output for and I don't know which model you are using. The I ran a Thermaltake for a year or so and it seemd a good PSU. The problem I DON'T like about TT is that they donot give a tempurature that the efficiency was rated at (will explain this below) The reason you need to find out your amp output is that a lot of 700's are only configured for about 40-48A. Still a 40-48A is ok according to manufacturer minimum BUT.....read on my friend.
nVidia SLI certified PSU's browse this link. Yes there are a few 650-700W PSU's on this list for dual 9800GTX.
However, something you REALLY should consider. If in fact your PSU is putting out 56A you are fine, BUT if it is only a 48A you should consider the following and possilby upgrade your PSU to a 56-60A.
The PSU is the single most important component in your system. according to EVGA the MINIMUM recommend psu for SLI 9800GTX+ is a 550W 30A PSU. so this would say that a 700W is plenty if it is pushing 30Amps. The amp output is the important stat to look at though you cannot get the proper amp output without the watts. So by all appearances you would be fine with a 700W PSU.
now why I suggest that use use a LARGER PSU.
You need to consider several factors, the age of your current PSU, the amount of extra components in your system such extra (more that 1) HDD's, fans(more than 2 or 3) / or LED fans, dvd drives etc... Manufacturer recommended minimums consider base tower essentials. each extra component of these pulls an additional 1-3amps except for fans, you get about 3 fans per amp except for LED fans there you get 1-2 fans per amp. so you need to use a wattage calculator to figure what your system requires.
after you have that the really important stuff comes into play like the age of your PSU. PSU's over time lose output ability as soon as 1 year old upto 5 years old your PSU WILL gradually lose upto 30% of its output ability. meaning that your 700W after this period may only be pushing 490W. your 700W is probably pushing around 40A. if its a 40 amp PSU when it loses that 30% it will only be pushing 28amps.
what does this mean??? it means WHEN, NOT IF, BECAUSE IT WILL LOSE THAT 30%, the question is how long will it take. when that happens you will lose/fry one or both of the cards and possibly your CPU or motherboard due to lack of power, and you RARELY get a lack of power warning. one day all of a sudden you will just start having problems and crashing. the longer you run like that the more components you lose. When PSUs take out components due to lack of power the video card is usually the first to go because it is the single most power demanding item in the tower, then the Motherboard or CPU is next.
How do you avoid this???? By learning and doing something that 99% of computer users have no clue is part of the power equation. Surprizingly most people who build there own computers have no clue about this either because one only finds out if they are an electrical engineer, randomly stumble on it while surfing the web, or by the experience of losing components because they did not know of the critical FACTS of a PSU. Which is how I found out. I lost a video card and motherboard within days of each other when my low quality PSU reached the point of losing enough output ability. I did not want it happening again so I spent hours researching what had happend and why.
use a PSU that is large enough to run your system while using NO MORE than 70% of its output ability at full load. such as if your system needs 35A at full load you run a PSU that pushes at least 50A. You want your PSU running at below 70% of ability at ALL TIMES. This means using no more than 70% when at full load, which also means your running your PSU at more like 50-60% under normal use. So to avoid frying any components you should use a PSU that will still provide enough power for you system even after it has lost 30% of output ability.
in order to achieve the above you need to look at a few things. obviously the amp output. but you also need to look at the efficiency rating of the PSU. the efficiency rating is usually 80% or above in most of todays PSU's, but you may run into some that are still in the 70% range. the efficiency of the PSU means how much of the power coming into the PSU from the wall socket is converted to watts to produce amps. So efficiency of 83% means 83% of incoming power is converted to watts, the remaining 17% is lost to heat in the conversion which is in turn the reason the PSU runs warm/hot.
but the efficiency rating is has another VERY IMPORTANT piece to it. You need to have a PSU that has the efficiency rated at 40C (40 degress celsius) or above. a lot of low quality PSU's will advertise a rating of 80% but when you look at the fine print it will say that was rated at 25C. This rating is total garbage, what they want you to see is the 80% but the FACT is your PSU WILL BE 25C a hour after you shut it down. Think about it 25C is only 77F. Your PSU in warm climates can be 25C if you havent used your computer for days simply due to ambient room temp. so a rating at 25C REALLY means that you PSU will run 80% efficient ONLY for the first few seconds it is powered on after that it will run at more like 70% or lower.
This means that your PSU runs HOT. Running hot is a disaster waiting to happen to PC components. 2 things kill PC components, heat and constant power on power off. constantly shutting down your system when not in use causes wear due to the expansion and contraction caused by the heating and cooling phases. A system will last much longer if left on at safe temps than it will if shut down everytime your not using it. Running HOT for long periods deteriorates the components, and if too hot will destroy them in a matter of seconds. So that means that a poor temp rating on the efficiency rating means your PSU is running HOT which will in turn speed up (shorten) the period of losing that 30%
Heat is also why you want to run a PSU at NO MORE than 70% of output. a PSU ONLY uses and puts out the power the system requires. so running at 70% or below allows the PSU to run cooler becuase it is not running at full load which means it is not producing as much heat, thus extending the period of time before you lose that 30%.
The last thing to know about PSUs is that the wattage rating that 99% of PSUs give, such as 700W is a PEAK power rating, meaning the PSU only puts out 700W when under full load. at normal load (referred to by PSUs companies as continuous power) it would be closer to 650W. PC Power and Cooling is the only company that I know of at the moment that rates PSUs at continuous power. Such as their 750W is a continuous with peak power of 825W.
This is all FACTS about PSUs
My next thing is a matter of personal opinion but backed up by a few facts. I HATE Multi-rail power supplies, and will only use one if I have to.
Multi-rail design is nothing more than a marketing ploy. It does sound good right? more +12V rails sounds better than one, right? Well that's exactly what the marketing is counting on. The fact is however that a PSU can only produce so many amps and multi-rail PSUs split that output among rails. For example you may have a 40A PSU with 3 rails. each rail rated at 16A. sounds good but it's really NOT. Each rail may be rated at 16A meaning each rail can put out UPTO 16A, BUT 16x3 is 48 NOT 40. and the PSU has a max amp output 40A meaning that if 2 of those rails are putting out the full 16A then the third rail can only put out 8A, due to that is all the total amp out put left in the PSU out ability. This also means that you CANNOT look at the rail amp rating and use that to determine the amount of total amp output. It will add up in a VERY FEW cases such as 2 rails @ 18A and a total output of 36A. BUT THIS IS RARE, You should NEVER count on adding up the rail output for a total output.
the next fact is that each rail is dedicated to certains area of the system, If your PSU has the same 3 rails at 16A and you do a system upgrade that now requires more power like one of those areas now requires 18 or 20A, You are Screwed and will lose the component from lack of power because the rail powering it can only supply 16A leaving you 2-4A under powered. This is the problem with multi-rail design because you may have a PSU that has plenty of amp output total to run your system, but the other rails CANNOT help supply that now under powered rail because they are dedicated to other areas. Multi-rails are limited to what they can supply and that is all they can do. Yes in most case they are fine, But if you add that upgrade senario you may find yourself in trouble and losing whatever nice new component you just upgraded to.
Single rail design avoids this. a single rail is only limited by the amount of total amp output of the PSU. If a certain area needs more power it is there provided the PSU has the amp output to supply it. and since your running a PSU at under 70% at all times chances of having that newly required power are pretty good. Though now you run into possibly running at above 70%, still you are powered safely. to avoid this run a PSU that will power you at no more than 50-60% under full load and an extra power demanding upgrade will keep you under 70%.
My personal opinion...PC Power and Cooling is the Best PSU on the market. Single rail design, they disclose ALL specs hiding nothing, each PSU comes with a test result. There are only 2 PSU companies that DO NOT produce multi-rail PSU's, this is because they know the truth about single vs multi-rail and are not trying to BS the customer. Those 2 companies are PC Power and Cooling and Zippy, Zippy is known for large PSUs, though they do produce a few for PC's they focus on large server PSUs.
If your current TT PSU is in fact a 56A the you are fine provided it is under a year old, if however it is only a 48A or more than a year old, I would upgrade your PSU to the PC Power 750W quad. It is a 60A single rail. or the 850W not that you need the power to run the system but for the safely powered purpose that one the PSU loses the 30% you will still be safely powered.
and bonus...the PC Power 750's are lest than half price right now at newegg. a year ago they were running $250+
PC Power 750
The last advise I would give is to stay away from off the wall PSU brands like Rosewill, apevia, coolmax etc...Most PSU brands are not made by the name they are branded. they are simply purchased having the manufac lable them. they do not care about quality, rather only quantity of sales. They choose a model that the manufac has engineered and run with it. Stay with know good quality brands. PC Power, Zippy, Tagen, FSP Group (fortron source) or highend models from Silverstone, Sea Sonic, Enermax, Antec, Thermaltake...someone who has made a name for themselves with quality PSU's.
Seasonic is a really good brand for its highend models and they actually manufacture some of the PC Power models to PC Power engineered specs. Then return them to PC Power for testing before they are marketed.
overall if you want a safely powered system when your PSU starts aging you should consider all the above. and remember if you want a good PSU your going to have to pony up quite a few bucks for it.
It is not about the watts but about the amps. Yes the watts are important because you cannot have the amp output without the watts but the watts can be configured to a wide range of amp output. you may have a 700W PSU configured to as little as 40A or as high is 58A.