Getting a gaming rig. Buy or Build?

[Epoch]

@ Paladin, holy shit bro, thanks for that.  That i7 processor is the one I was looking at getting, its got a combo with

http://images17.newegg.com/is/image/newegg/14-131-185-TS?$S125W$) no-repeat scroll 0% 0%;">  

1. POWERCOLOR AX5870 1GBD5-MDHG Radeon HD 5870 1GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card 
for $650.  Seems like thats a good chunk of the budget. 
I like that power supply, I'm guessing 750W is enough to run the PC just fine?