Look at second hand stuff on eBay, you'll be able to get yourself a GTX260 or 280 for almost nothing now which will be a MASSIVE leap up from what you have now - an 8300GS is pretty much the same as the Intel integrated graphics
You should have a nice balanced PC then even with your (I assume) Q6600, you should be able to get 3Ghz out of it pretty easy with a bit of tweaking although be prepared for some monster heat output if you go much past that!