I have an Athlon 3200+ XP - will i be able to enable it?
I have that same processor. The Barton is one of the better single cores out there, so you should be fine.
How much does the option affect gameplay? Is it much harder with this option enabled?
I honestly can't say. This option came out with a number of other changes, and the game has changed even further since. Not meaning to be 'flip, but try it and see for yourself.
difficulty affects the starting bonuses of enemies, right?
The AI is only given bonuses in the settings above 'tough'.
What determines AI performance if not CPU usage?
I don't think it's so much a matter of cpu resource allocation as it is the algorithms that the AI is allowed to use. (Except, of course, the 'max cpu setting that we are talking about.)
Why can i enable "force max cpu" while setting my enemies up with the intelligence of a slice of bread?
It's the flexibility of the game. We are given any number of settings that we can tweak to get the game experience that we are looking for, and that's just part of it.
do very high intelligence settings automatically mean drastic cpu usage?
I wouldn't say drastic, but I'm sure that the higher settings use a bit more of the cpu.