Here is a link to Tom's monthly list of best video cards for the price. The septemeber compilation should be out soon. As this is August the prices may have shifted slightly, but in general its a good judge of price/performance. Importantly it lists what resolutions each recommendation is good at. So if you're monitor limits you to something under 1920x1200, thats something to keep in mind.
To maximize the life of that quad, you could go with a 1gb radeon 5830 or a Nvidia 768mb gtx 460, both of those are around the $200 range, I would only recommend those if you plan to keep your rig for around another 2 years. Otherwise a Radeon 5750 or a 5770 would probably be better values for you in the near future.