The guys at THG run seventeen different DX10 cards through four different DX10 games at varying resolutions & graphics quality to see which one gives the best performance under Vista. I'll give you a hint: it's the expensive one.

"While their performance lead over the Radeon 2900 XT and the Geforce 8800 GTS was not as pronounced under Windows XP/DirectX 9, they offer between 50 and 100 percent more performance than these cards under Vista/DirectX 10."