Avoid the 7000 for anything but basic graphics applications. I bought that one for my first homebuild and it was obvious within days that it was my system's bottleneck. Your system will do nicely with an nVidia 6600GT or 6800/6800GT 4x/8x AGP card - I tried both in my system at the time and the improvement was obvious and immediate. (I wouldn't go with any 7-Series or 8-Series cards, they were designed for PCI-E application and backward-engineered for AGP boards but that's just one guy's opinion.) If you go for the 6600, RivaTuner can be used to unlock the 4 locked pixel pipelines to give you the same performance as the 6600GT for a few dollars less. I preferred the 6800GT for the improved performance I got on some games at the time.

You can still get them on eBay, sometimes. I think the only 6-Series card left being made is the 6200. I have one 6800 left, a used 6800 Ultra with 256MB RAM and a Zalman cooler. Darn thing takes up two expansion slots on the board but it's a sweetheart.