You can check in your case, or online. I took a look, and it appears that you only have a 220W PSU, which is barely enough to run what you have now.
If you want a really good graphics card, you are going to need a minimum of 500 watts or more (depending on card).
If money is not an object, best single core chips on the market are the 5870 and the 465(?)
PassMark Software - Video Card Benchmarks - High End Video Cards