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.