Just as an example, I just spent about $950, and built the box I am on now. Specs follow.
2.6ghz AMD64 X2 proc.
1 gig Corsair Ram
SLI capable board
I went with the 7600 GT nvidia 512vid ram
2x 80 gig WD SATA 3.0 harddrives in RAID 0
Zalman HS/FAN
Gaming Case w/4 fans
granted I did already have XP pro myself, so I didnt have to pay for it. Still alot cheaper than you will get that AlienWare machine for. Its like buying a Dell. You pay double for the name.