Right as i have had a 5870 and now have a GTX480 i can comment on this, dont listen to what people say about heat or power because i ran my 5870 off a 500W psu and now im running a GTX480 off a 500w psu (no problems so far) and the they are both about the same noise wise when gaming.
All graphics cards get hot hot when gaming and the 5870 is no exception as it gets very hot also, just like the GTX480.
My 5870 was good dont get me wrong but after trying the GTX480 its a much better card, the drivers are miles better... i mean im running the 256 betas and they are more stable than any of the whql of ATI's i tried.
Another plus is the games just feel that bit smoother, i dont know why but they do. Plus you get to use more eyecandy and i can only feel these 480s have a lot more to offer.
If it was me and i had the choice from the start i would have got the GTX480, but i am glad i had both to compare.