The extra cache on the new chips is good for another 10% frame rate boost, but you get little to no frame rate gain from the additional two cores, as pretty much all the multithreading enhancements were done only to the terrain loading engine.
In addition some extensive benchmarking was done over at avsim, and it was noted that a 50% CPU overclock translates to a 34% performance gain, which is an efficiency factor of 68%, on both XP and Vista.
But yeah, the CPU is where you want to invest your money for FSX. Any GPU faster than a 7800GTX will easily do 1920x1600 w/AA and not see a performance hit, even on the fastest CPU. Toms chart shows a 2fps difference on the 7800GTX going from 1024 noAA to 1920 w/AA.