I have a different mobo and unfortunately it only supports AGP cards as well! I'm running ATI 9800 PRO (128MB) graphic card and it's pretty decent although I'll be changing my Mobo and Processor soon! Btw, the 9800 PRO has no DirectX 10 support and no Shader Model 3.0 support. You could try the 9800 PRO.
The 6300 wont give you great perfomance any game with midrange GFX. The 7600 GT or 7950GT would give you a good level of perfomance, although i feel u would have to add some extra RAM to take full use of either of them.
Also check the power rating of your PSU as u dont wanna blow it up :).