The problem with the non K ones is that they are fixed. They only overclock via their turbo feature which is very limited when it comes to the 'real overclocking' the Ks can do. So, if you can cut back on other stuff to get a strong CPU (FSX is very CPU dependant), it may be the best solution.

The AMD stuff is really good and in other games the 6 cores may help there, but FSX is an old architecture, eating up even overclocked 4 gigahertz and more CPUs. So if you can get 4.5 or so with a K model, you should be very ok.