I believe FSX will only use up to 4 cores but that does not mean 6 would not be beneficial in other ways, just that it won't help much in FSX. Also, with the exception of the latest generation upper end ATI cards, Nvidia tends to run FSX better especially when it comes to autogen shimmering.
You will get the best results by going with a 64 bit OS and setup is critical. An excellent starting point is to read NickN's guide at SimForums. There is also a new thread with instructions for Win7. Also, you can find details here as to why Nvidia will generally give you better results. Like many others, I have used his suggestions and been very happy with the results.
fsx loves a high clocking cpu.. if you are not overclocking you should go for the highest cpu clock you can get, nomatter 4 cores or 2 core.. i do think 4 is a better choice. but a 2 core with a higher clock speed will do bettter..
need at least a mid range gpu last guys post was correct but the res quality of the ati is better then nvidia fps vs res....? always hard to choose
one of the best bangs for your buck would have to be an ssd drive as fsx needs info quick. it was the best fps gain i saw on my setup over all hardware upgrades i have done...
keeping the computer running cool is a must.. you will take a hit on fps with a hot cpu.. keep its temp down...
E.C.O water cooler 80$ my temp has never gone past 55c
gigabyte/2oz copper boards i like..
ati 5770 or higher
SSD drive... O/S on the same drive as the fsx and all its addons, try to keep the computer looking for all the fsx info in one place..defrag defrag