Since you mentioned 6670 and 6770. My advice is to get the 6770. Swifty morgan is right that a 6670 is almost the same as the gpu you have right now. The thing is.. Your power supply might be a problem. The AMD site says that 450W is the absolute minimum, I would not get too close to that minimum.
I have read a couple of guy's opinions that even a 400W PSU is fine but better safe than sorry I suppose. So there would be no configuration problems aside from maybe the power supply?
I was wondering whether the motherboard could support a 6770? The bios is updated and everything.