Even a HD4670 may be bottlenecked by that CPU. A HD4650 might be a better choice, particularly as it would put even less strain on your questionable PSU. Most HD4650s are DDR2, but you want one with the faster DDR3. Biostar has one; I just installed it in my wife's PC this weekend and it was painless.
You might also get some performance improvement by going to 2GB or even 4GB of RAM, but DDR2 is getting more expensive. It might make more sense to save for a new rig.