Supreme Commander is the shit! Loved that game. Unlimited units is a must for that game.

I found it too slow paced even on the highest speed settings and edited one of the game files to increase unit and building production by like 5 fold. Made the game so much more hectic and fun (since the computer also benefits).

FEAR is also a good game, with everything maxed out and with Shadows on their highest settings it brought my computer to about 16fps! (16 aniso, 4x AA @ 1680x1050). Not bad for a fairly old game. Could be the gfx engine is not that optimised.

UT3 is cool, but that gfx engine is killer and will run on pretty much any system. Always loved the Unreal engines.