I actually built a pc this week for 900$ budget for my friend..it ended up 850$ due to reason below, here are th specs:
core i7 920
RD4890 OC ATI
3gb PC3-15000 triple channel ram (he didn't have a 64bit OS and we decided to wait till W7 comes out otherwise I would have gotten 6gb)
MSI X58 PRO-E
600W OCZ power supply modular (The 750W were out of stock
..but 600W should be plenty anyways since he doesn't plan to CrossfireX )
Thermaltake case
1tb HD Western Digital
22x samsung DVD bruner