Assuming your gpu is a high end $400 above card, the FX-6300 will help significantly to maximized the expensive gpu.
But if its only medium class such as 7950 or around $200 card, then the difference is minimum if you have overclocked your x4-B55 into 4GHz like mine.
You should overclock your cpu at least into 3,6GHz with its original heatsink (increase voltage into 1.45V and multiplier into 18). I have done the test 5 years ago and still using it now with 4GHz (1,5V multiplier 20) with additional heatsink deepcool iceedge 300 that only cost me $15 5 years ago. New model of the heatsink is deepcool Gammaxx 300 and should be less than $20 and it can be used almost any model of cpu except the latest intel 2011 socket.
Hope it help