By coincidence, I used to own an nVidia 7600GS in my last PC. I bought it because it was the bare minimum required for supporting Aero with Vista at the time. Things have moved on a bit since then, so I'm sure you would benefit from something a bit more powerful.
However, before upgrading, have you tried updating your graphics driver. I have seen different graphics drivers make a HUGE difference in my time.
As to the difference between nVidia or ATI, I have owned mid to high range versions of both. The only significant differences, in terms of performance etc, are only really apparent when you are dealing with high end graphic apps and games. For a Windows app with no special GPU accelleration requirements, I would say you would be fine with either. I'm using ATI at the moment with no problems at all - in games or in Sonar.