Wait for Nivida's DX11 card, Fermi, they have added architecture to deal with the new code implemented in dx11. ATI 5 series cards take a huge performance hit in the DX11 game Dirt2. Maybe the next game will show better performance. Something tells me it won't.
Not all games are like Dirt 2, in Stalker I get more FPS in DX11, than I do in DX10 on AVG.