[{HVY-E}Jinxx]

Just enabeling it may not do the trick. You have to tell the card to over-ride the game.
Also, did you run Driver Cleaner before you put in your Nvidia card to get rid of ALL instances of those garbage ATI drivers?

If you don't know what I'm refering too, shoot me a PM and I'll walk you thru step by step.
I'm currently running twin GTX 295's with 62fps constant (I leave Vsync on due to Track IR limitations). With Vsync off I get between 144 and 250 fps.