Minimum for gaming would probably be a 5670; maybe higher, depending on your resolution.... it would have to be awfully low to get away with it.
I'm running a 5770 right now, and it has major troubles on max settings at 1080p for newer games. I'd say for actual gaming it's the starting point. It all depends on your resolution - For 1280x1024, you can do with a 5670. If you're up into the 1600x or 1920x1080p range, then you'll want a 5770 if you're going to play newer games.
As has been said, the 5470 is basically for multimedia - watching movies, videos, etc.