Does your motherboard have PCI-e 16x slot available? if so, are you going to game a lot? if so you could get the HD3850 for roughly $180 (newegg) but if you want to be able to play games at high resolutions no problem you should wait for december when prices come down and look at cards like the HD3870, 8800GT or the new 8800GTS that will come out. But if you are on a budget ($150-$200) then the best way in my opinion would be the HD3850...
Well, you could (1) open your computer case and look into the motherboard, or you could download the program called "CPU-Z" its free, and it tells you everything (almost) about your computer hardware...