I have just upgraded my video card from a low range one FX5500 to a mid range 7600gs. About $180 Australian. I am happy with it and still managed to stay within my low budget. It all depends on how much you have to spend, Do you prefer Nvidia over ATI or vice versa, what you want to do with the computer. Are you playing high end games or simply using it for the internet and letters. Also is your present hardware going to handle a high end card, AGP or PCI Express. These are things you need to think about as well as how long are you planning on keeping the computer or are you going to get a new one in the next 12 months or so. If you are a serious gamer then i would concider waiting til vista and directx 10 is released as new videocards are going to be out soon that are designed around this technology. First thing i did was read a lot of reviews and i mean a lot. Here is a good start -
http://xtreview.com/review140.htm Then got the best card in my price range that would meet my critiria.
Best of luck with your choice Allen