so far, I have tried few amd cards (4xxx and 5xxx) in Maya, and they all suck at hardware rendering. they do render a lot faster but i get 'snow' on the images. i believe ati is only good at the firepro series. maybe the new 6xxx series will do better, but its a gaming card, so dont expect much.
do some search about nvidia cuda, from what i see nvidia is doing far better on that field.
Nvidia cards are better with applications, especially when it benefits from the use of CUDA. Although AMD seems to be trying to balance gaming and application rendering, a firepro or quadro will be the best for 3D applications to run smooth and fast.