... I've been a Flight Simulator enthusiast since 4.0, so I've always designed my rigs especially to run Flight Simulator. No "games", just the "simulation". FSX SP1 & SP2 are coded for multiple cores, so FSX will use as much raw CPU horsepower as you can afford to throw at it.
Frame rates scale nearly 1:1 with CPU clock speed, so if you overclock from 2.4 Ghz to 3.6 Ghz, frame rate will increase nearly 50%. Additionally, I've tested FSX at 3.6 Ghz on an E6600, and at 3.6 Ghz on a Q6600. The increase in frame rate is nothing less than an astonishing 80%.
A quad with a high-end cooler and a high overclock is essential for the highest frame rates in FSX. Google the Flight Simulator websites for the best fsx.cfg file tweaks to achieve the best balance between frame rate and image quality.
For graphics, I recommend the 8800 GTS 512. It's under $200.00, has enough memory for high resolutions, and enough horsepower for running GPU bound "games" beyond running the CPU bound "simulation". Equally important, the 8800 GTS has dual slot rear exhaust which maintains low case temperatures to keep an overclocked quad running cool...