It's hard to go wrong by trusting Tom's Hardware. They're a great site, and here's a good article that I'm using to determine my next video card:
http://www.tomshardware.com/2007/03/...ney/index.html
This page, specifically, will give you an idea of the video card hierarchy and what nvidia product is equivalent to what radeon product and vice versa:
http://www.tomshardware.com/2007/03/...ney/page7.html