Give
www.ibuypower.com a try. I haven't used them myself, but I kind of regret not having done so.
I bought my current machine about a year ago (meaning it's about a year out of date) which is a Intel Core 2 Duo E6600 with a GeForce 7600GS. I actually bought it from HP because at the time I didn't know about Ibuypower and didn't want to buy individual components from Newegg and put them together myself.
I discovered Ibuypower after I bought my new PC. I got a decent deal from HP but for the same money I could have had the same machine with the Geforce 7900GS instead of the 7600GS. Oh well. Live and learn. However the biggest thing I would have saved is that HP bundles it's machines with tons of crapware that I spent quite a bit of time stripping off my PC whereas from Ibuypower you basically get a vanilla OEM restore disk.
Anyway they have configurators for Intel Core 2 Duo, Intel Quad Core, AMD Athlon 64 X2 and AMD Athlon 64 X2 CrossFire systems. They take you through all of your options from case, PSU, CPU & Cooling, Motherboard, RAM, Video, Hard Drives, CD/DVD, etc. Plus you can get it without an OS or with Home/Business/Ultimate Vista or with Home/Professional/Media Center XP. My personal recommendation is still XP Pro, but to each his own.
As far as system recommendations I wouldn't hazard a guess since my experience is about a year old at this point. Good Luck.