DirectX 11
should be better than DirectX 10 performance wise, although I can't really say much as I haven't seen how smoothly games have been said to run under DirectX 11 nor seen a comparison to DirectX 10; in other words a benchmark. Tesselation is one feature I am looking forward to seeing (after looking at the Unigine demo and DiRT 2) as it will bring a high level of detail to games should developers use it.
I'm with Striker on this one; concerning the GTX 4xx series. Despite nVidia having the GTX 4xx series which includes more features than the Ati counterparts (PhysX, CUDA, etc), I doubt a price tag $200-300 higher than the Ati 5xxx series is going to have the majority of consumers buying it, let alone want them to have
a jetplane
in their PC. Until nVidia drops the price on this, reduces the amount of heat it creates and somehow manages to reduce power consumption, Ati in my view is currently leading the market in terms of DirectX 11 GPU's; more affordable and more importantly doesn't run like a jet plane.