You're really not going to get that much of an improvement just by upgrading the CPU. If you are not satisfied with your current gaming experience, upgrading the CPU and Graphics card will improve the performance but if you really want to get the best performance you should consider saving up your money and building a system with new components. Since you claim you are an AMD fanboy, the "Piledriver" series of AMD FX processors is quite good, nearly matching the performance of the i5 processors from Intel. For a good gaming PC, I would recommend saving up about $800-$1000 and then just building a new PC. You could convert the older system to a HTPC or just donate it to a charity that accepts old PC's.
Edit: corrected typo's