Just need to chime in here...

I upgraded my 9000pro to a 9700pro recently.
With the 9700pro, it looks like a brand new game. However, the fps did not seem to improve very much, though slower aircraft run as fast as the default aircraft now. Really it is just more consistent.
Specs: P4 2.1ghz
512mb PC2700DDR
At least 5 gigs left on HD
Running XP pro, 1280x1024 res, 32bit textures, all sliders maxed except for autogen and scenery complexity, which are at "dense"

Now, I get generally high-teens FPS, dipping extremely(to about 10-12)near larger cities and airports.
In conclusion, I think that we need to locate what takes over near the large airports, be it RAM, CPU, or video card.
(sorry, just got back form my English midterm

)
P.S. My benchmark is approach into Heathrow facing the city, in VC.