I think the GF4 is above and beyond the GF3 and the ATi Radeon 8500. Between the GF3 and the Ati its a toss up. Personally I go with the Gf3 because of Nvidia's excellent driver support. They are both very fast cards with excellent imaging. I feel that Nvidia's AA is better than Ati's.
The GF4 beats all the current available video cards in all areas in terms of specifications and performance. The real "battle" is between the R8500 and the GF3, I think. The GF3 outperforms the R8500 in some areas while the R8500 outperforms GF3 in others. Once ATi actually releases some BETTER drivers (the leaked one's are ok), the R8500 might pull ahead of the GF3.
On a more personal level, I'm quite content with my R8500.... though I'd be even more happier if I had better drivers.
Some of the related links you can check out that has some opinions on this:
I'd say right now the choice is very difficult to make (unless you'd go for a GF4, of course). The GF3 and the 8500 are rather on a par. Of course, Radeon has several advantages (dual display etc.) and it looks more futureproof actually. It's also cheaper than the GF3 Ti500. And ATI's drivers HAVE matured finally. But nVidia should be a little more reliable. Tough choice...