In order to answer this we will also need your power supply information, and your motherboard specs (is it an AGP slot?) If you dont know just post a link to your manufacturers profile for your system.
As deadlockedworld said, we'd need to know which slot type you have (AGP?). It would also help to know your power supply specs - not just the wattage but also the amperage ratings for the +3.3v, +5v & +12v rails. Of the three, the +12v is the most important. But assuming you have an AGP slot & a good enough power supply, you might wanna consider this Radeon X1950 Pro for just $65 at TigerDirect: