The HD5570 looks like a decent low power, low heat option. Make sure it comes with the low profile bracket.
There's an article / review of the HD5570 here:http://www.anandtech.com/show/2935
It certainly won't be playing Crysis, Bad Company 2, or things like that. But to play World of Warcraft or something simple like that it should do fine. It managed to play Left 4 Dead @ 1920x1080 at max quality to my surprise. http://www.anandtech.com/show/2935/12