Start out on ebay and you might be able to find a faster CPU + cooler, better AGP card and more RAM but that's going to cost you anyway. Fact of the matter is that even if you were able to OC that CPU (and you can't) it is not necessarily something you should do because you probably don't have very good cooling inside that prebuit case meaning you stand a fair chance of destroying the HW you do have and ending up with no PC at all.
If the game is experiencing some kind of glitch rather than just low FPS then you should take a look in your logs to see if you can find the reason.