The power supply needs to be upgraded depending on the card you want to get, because otherwise your computer won't be able to handle the new pressure and keep overheating.
There are the new DirectX 10 series coming out that are at or around $100, and some even cheaper. I've heard really good things about the 8600 GT, and it's really the best bang for your buck at the moment if you want a real nice boost in performance.
Of course, I'll have to know what your budget is and what kinds of games you'll be playing to give you a much better suggestion.