The answer is really simple: X Plane is CPU limited with the NVIDIA cards. There are probably driver optimizations that AMD has that makes it run better, or perhaps the app is using a different path on NV.
As far as I know, XP is known to be very CPU intensive, which is why you're not seeing any change in FPS when you crank up the settings.
No, not always. On modern computers, XP is seldom CPU limited, although that is to a degree user dependent upon selected settings. GHz matters most, not cores, unless, for example, you run a bunch of additional aircraft at the same time, which may be fun for gamers, but is pointless for professional use. The mobo is also seldom a limiting factor. It is most often the GPU. Frame rates I am getting on the 670 are significantly faster than those reported by 5870 users who use the same/similar rendering settings, hardware, and other XP options. There are so many to choose from that hardware comparisons are almost impossible unless the built-in command line framerate time-demo is used. Unfortunately, that too is a bit buggy in the current release.
Also, at least in XP, the GTX admittedly runs better under Windows than ML. In OS X, Barefeats reported significant XP10 FPS drops in ML compared to Lion too. With my new W3690 CPU, I've yet to see a single core hit 100% In fact, I haven't seen a second core above 30, and the others just sit there. Again YMMV depending on your settings.
According to Barefeats, at low screen rez, the 5870 absolutely spanks the GTX 5XX, but at high rez, the MVC GTX 570/580 are slightly ahead of it. Obviously, with improved drivers, the 670/80 will be even better. For me it definitely already is. XP is a conundrum though. If you can solve it, maybe LR will hire you.
See:
http://barefeats.com/gam12.html
Bottom line: No computer on Earth will run XP with all of the boxes checked and settings at max. Where the fault lies is another matter.
