I have a biostar board with sempron cpu. Have you tried overclocking yet? Yesterday, I dropped my ram speed from 1333 to 800 and raised my cpu fsb to 270 for a decent overclock. Your ram may be different, but give it a try. When you lower the ram speed first, the speed increases when you raise the cpu fsb so the net speed is close to specs. My ram speed now is only 540 in cpuz (1080) but the board set the timing numbers lower, so the whole system feels faster. For a cpu upgrade, check for a four core phenom II in your price range. They are being phased out now. You may find one used on craigslist for under $100. Here's your cpu support page: http://22.214.171.124/app/en/mb/introduction.php?S_ID=436&tab=1.
Here's a good cpu for upgrading: http://www.amazon.com/AMD-Phenom-Processor-Black-Socket/dp/B0056D5AMY/ref=sr_1_150?s=pc&ie=UTF8&qid=1326635065&sr=1-150