Remember that, according to skuzzy in some thread in here somewhere, AH uses different rendering paths that take advantage of newer generation card features for more advanced effects. So while you may get almost the same framerates with a 9800 and an x800, chances are you're also seeing a slightly different and better image with the x800. It's also been my experience (with a 6800GT) that with the newer vid cards you can turn on anti-aliasing which significantly improves image quality, and use the larger texture sizes which also has a dramatic effect on image quality.
3dfx learned the hard way that framerate isn't everything, and AH2 seems to prove it. 40 fps with a 4200 and 40 fps with an x850 or 7800gtx are not at all equal in image quality. This is true almost regardless of what cpu you are using.