Just googled a bit and it seems the 4670 only draws about 15W more than the 4550 under load so I guess it could also work on a 300W PSU. The performance gain compared to a 4550 is about 2-3 times more FPS, which means many games would be playable at resolutions above 800x600 :-/. You can see the difference here
. However, note that ATI recommends a 300W PSU for the 4550 and a 400W for the 4670.