I would HIGHLY recommend you read my FSX thread Devrij mentioned, in there I posted a few links on how to improve FSX performance. Many of these links will also discuss what hardware suits best for FSX as you need to consider CPU/GPU/RAM/HD/SSD/OS etc. So before you splash out your hard earned cash, make sure you know what you are getting and know how to get the most out of FSX. It can be a beast and a source of much frustration but once you tame it, it can be a real pleasure to use for many years to come.
Best of luck.
PS as far as multicore usage goes when it comes to FSX: upgrading and patching FSX with either both service packs or Acceleration (which includes both packs) is sound advice. It gives many improvements one of them being that it brings in 'limited' multicore usage. From what I have read, the patches makes some of the processing of FSX (like autogen) work on a second processor if there is one available. Again from what I have read, the multicore implementation is not up to nowadays game's standard, AFAIK FSX only uses 2 cores maximum, it is other applications/resource or addons that would utilize the 3rd and 4th core. So yes it uses multicore but it still relies heavy on RAW CPU power, I see no evidence atm that two identical PCs, one with a 4Ghz Dual core and the other with a 4Ghz Quad core will run FSX any faster. The ONLY difference would be what else you run on that system at the same time as running FSX that would benefit from a quad core so it would be less likely to interfere with FSX needs for CPU cycles. IMO going for the highest clocked quad core would ATM make the best sense (you could save money and get a similar clocked dual core but then you have to be very careful on how you tweak FSX and what other processes you run while using FSX - I tend to use a 2nd PC via both widefs and simconnect to handle several FSX 3rd party addons in order to take the load of the FSX PC plus I keep the FSX PC very very clean in term of processes and resources). ATM I cannot see any sense in spending money a more expensive CPU that contains more then 4 cores, again, unless you are doing more then just FSX on that PC. Hope that helps.
