Neal Stevens
Fallout3 using the same game engine as Oblivion, as far as i undertand. Not that heavy game.
If you realy want to test performance try to get.
Crysis on dx10, a heavy game.
GTA4 - (not for kids) I think the developers push it too far thise time
but its still a very good game with lot of stuff to do. Addicted as well, at least its my opinion.
Any way, have fun with your new PC
