Your system will be able to handle any cards pretty much. It depends on what games you intend to play.
Radeon 4670: about $70. Capable of playing most modern games on playable framerates at lowered settings.
Radeon 4850: about $130. Capable of playing most modern games on decent settings. The 4870 is better.
Radeon 5850: about $300. Play most modern games on high settings.
The above is just a general idea. It would help to know what games you intend to play. If you don't game, anything 4670 and below will be more than awesome. If you don't do anything graphics intensive then the onboard graphics are good. If you intend to get a good card, you might want to consider more than 450W.
So what do you intend to do with the graphics card? Game? If so, which games?