THERE'S SO MANY!
Team Fortress 2 you can't pass up this game
Cod 4 / Cod 5 both exceptionally fun. Great graphics for your new beast too.
World In Conflict, if your into Modern Combat RTS (OR RUSSIANS).
and of course... Company Of Heroes, mind blowing graphics and great World war II RTS.
All of em great games that would play well on your new machine.