You play a lot of AoC, yes?
I've compared the 470 and 5870 @ stock in that title.
Here's a very demanding wide shot near Kesh, 4xAA|16xAF all on or enabled/max @ 1920x. I use DX9 because 10 is somewhat buggy regardless of card used.
470 = 42FPS
5870 = 36.6FPS
Both are well above the 30FPS threshold and perfectly playable. Load for the 470 was 81C and load for the 5870 was 65C. IMO the FPS edge of the 470 in this title is offset by the much cooler load temp of the 5870.
You'd be happy with either card and the 5870 does idle and particularly load cooler.