Tough call... It'd be close, especially with any overclocking on the CPU or 8800GT, and dependent upon additional devices. An 8800GT draws almost 2.5x the power of a 2600Pro at full load. That's quite a difference.
It might be better (safer) to go with a 9600GT instead. You'd lose a little performance compared to a 8800/9800GT (they are the same card), but it draws only 2x the power of a 2600Pro. If you think your system can handle the power requirement of an 8800GT, wait for the ATI 4770 to come out in a couple weeks. It's priced at $100 and ATI claims it beats the 8800/9800GT. Current wattage ratings for it show as 80W, identical to the 9800GT. Could also consider a 55nm version of the 9800GTX+, also rated at 80W...
I just don't know if your system is actually capable of sustaining that level of power. If you're getting this 8800GT super cheap, I say try it, but don't be surprised if you see system crashes in games while it's under load.