I just recently built a new gaming PC and used an Intel Core 2 Duo E8400 3000mhz processor. I thought the CPU would be powerful enough to run FSX with almost everything maxed out but I was wrong. So knowing the E8400's spectacular overclocking potential I bought an aftermarket cooler and proceed to overclock it.
I went into the BIOS and immediately set the FSB Multiplier to 400 which yielded in a 600MHZ increase in performance and a higher FSB speed of 1600. The great thing was that with this processor I did not have to increase the voltage to reach 3600mhz.
I was granted a good performance increase in FSX, I can now actually run with 20% road traffic and 40% boat & ship traffic along with Anisotropic filtering and Anti-Aliasing with silky smooth frame rates.
I might try to hit 4000mhz sometime in the future when I get some new case fans and a new cooler for my GPU to keep overall temps down within the case.