With AH, you could get rid of some texture shimmering by going full anti-aliasing and ansiotropic filtering at 1600x1200. That would look kickass and probably actually give you a decent framerate with a 9700 in AH.
A GF4 just can't get all three, antialiasing, ansio, and resolution, without suffering a big framerate hit. The ATI 9700 can, but of course it costs $300. I wouldn't personally consider it worth it upgrading from a GF4. From a much slower card, maybe, but not from a GF4.