Title says it all really. i7 has more cores but is 2.0 GHz i5, is 2.3GHz and I have heard that for FSX a faster one is better regardless of cores (both can be overclocked to 2.9GHz but I am reluctant to do this). Thanks,

Those chips do not have the same number of cores so the clockspeeds are not directly comparable. The i7 is a true quad core chip whereas the i5 is a hyper-threaded dual core chip. You will likely be happier with the i7, especially if you overclock.