Start with a Q6600 (or Q9550 if you can afford it) and a Xigmatek HDT-S1283. Performance in FSX depends a lot on the CPU, and an overclocked quad is your best solution. The video card is less important in this game. Something like HD 4850 or better would be fine.
as aevm said, it's all about the cpu. for flight simulator, definitely get a quad, the fastest one you can afford. any P43/P45 board should be fine. As for gpu, a 4850 is more than enough. you could probably get by with a cheaper one. and definitely get 4GB of ram. fsx needs alot of memory. to make the most use of the memory, make sure you get a 64-bit version of windows.
It doesn't really need a quad core just speed. Quad isn't gonna hurt but it runs just fine on my 3.2 dual core. All settings set at there max. I couldn't do that with a 2.8. I am using two 4870's in crossfire and running it at 1900x1200. With Vista Ultimate 64.