The 6800GT is much faster. (since it has 256bit memory plus 16 pipelines)
A 7600GS is similiar to a 6600GT in performance. (About ~5% faster).
Sounds like hes upgrading his AGP system. Plan A is to try looking for a cheap 6800GT/X800XT although its hard to come by.
However you have plan B too. Wait for 7600GT AGP (which is identical to 7600GT PCI-e and faster than a 6800GT), or the X1650XT (R560 + AGP). These cards will offer better performance, be smaller, less heat/power while giving you all the latest features like pure video/AVIVO.
But seriously, plan C is better. Save money and move to PCI-e since the current graphics cards are going have another price cut.