BTW, is i7 2600K a better choice than i5 2500K in this particular case of FSX?
Won't make a difference, buy the i7 2600k if you do a lot of video converting and use programs that can use more then 4 cores/ threads. If you only game and do light tasks on your PC the i5 2500K is more then enough for FSX, you won't notice a massive difference from the i7 to the i5 also you will be overclocking the i5 2500k anyway to gain a few more frames out of the system.