If you are considering a 5570, make sure you get the DDR5 models, looking at 10%+ better frames for same price (compared to DDR3). I have one in my HTPC and its perfect for 720p gaming (my HDTV is only 720p), DIRT2 on high @ 2xAA and steady 30fps, low power draw, pretty quiet.
You can get them passive too if noise is important to you.
If your rig and budget can handle it though no question 5770 is the way to go. If DX11 and eyefinity is not important I'd just snag a used 4850.....