I don't disagree but since he wants the computer for gaming:
DDR2 is a lot cheaper and won't give him a difference at all in gaming.
As far as hard drive space, he can ultimately choose the capacity.
He does not need 650W, heck even 500W is enough. And my suggestion is modular.
Dual core is cheaper and better for gaming, period.
Anything bigger than 3-4GB of RAM won't affect his computer unless he's gonna do some heavy picture editing.
1066 vs 1333, same as 800 vs 1066, really no big difference. Although you win on this one