Wait- free or paid?
Warrock is a free FPS:
Commander and Conquer: Generals is still a great game, even if it's a bit outdated.
You could run Counter-Strike:Source, Half-Life 2 (barely), Day of Defeat: Source etc. Check out the steam games at steampowered.com You can buy them all online.
Also, to check your system specs, you can go to start --> run--> type dxdiag --> press enter