What is your price range? Also, what is the make and model number of your computer. If you have details about your power supply, and your motherboard, please list them. We'd like to know how many total watts the power supply has, and how many amps on the +12V rail(s) (ex. +12v 18a). The label on your power supply should show this information.
Besides all this, if your power supply and motherboard can handle it, I'd recommend getting an ATI Radeon HD 4850. Currently, this card has the best price to performance ratio. It is usually $200, but some might have rebates. If your looking for something better, the ATI 4870 just came out and it is really good also. At $300, it beats the Nvidia GTX 260 which costs about $450.
Now if your looking for something cheaper, the Nvidia 8800GT is pretty good.
Here are the latest benchmarks from tomshardware.com http://www.tomshardware.com/reviews/radeon...870,1964-6.html
"I think there is a world market for maybe five computers." - Thomas Watson, Chairman of IBM, 1943