I think you can get 5670's that rely only on PCI-E power.
However to game with those at okay framerates, you'll need a pretty low resolution monitor at low settings.
I have a HD 4650 that I can run most games (bioshock 2,left 4 dead 2 for example) at 1920x1080 medium to high settings
Crysis warhead runs at 1280x1024 mainstream settings
the HD5670 is better than the HD4650 so will run most games in the past year or two decently
Toms picked the HD5670 as top pick under a $100 bucks for gaming