You can get X2s for skt939. However they are becoming more and more scarce. You can still find Dual Core Opterons (which are similar) for pretty good prices.
A X1650 is a pretty decent card for the money. See if you can find a 8600GT or even a 7900GS in your price range.
However, for what you use your computer for, I would leave it the way it is. The performance increase that you will get from upgrading your CPU doesn't make it worthwhile, IMO. Save your money for some bigger upgrades, or spend it on something else