Go the biggest and fastest you can find and afford
Below are some of the components of the system I recently put together:
CPU Intel Core 2 Quad Q6600 2.4GHz 8MB cache
RAM Kingston KVR667D2N5/1G 2 x 1GB 667MHz DDR2
Hard Drive Seagate 320GB SATA drive
Graphics XFX 8800GTS 320MB 1.6GHz PCIe
Monitor Acer AL2216WD 22" Wide
Now this system still struggles to run the current version of FSX (with SP1 installed) in the monitor's native screen resolution of 1680x1050, but I have not tried a lower res as yet. I am hoping that SP2 and eventually the upgrade to full DX10 will bridge the gaps and make this as playable and enjoyable as FS9, but with the extra eye-candy that FSX delivers
If these updates don't make a great deal of difference to performance I have left enough upgrade scope with my base hardware to go to an Intel Core 2 Quad Q6700 (2.6GHz) and more importantly the 8800 Ultra graphics card. I would also consider going up to 4GB of the current RAM, but not sure if upgrading totally to 4GB of the 800MHz RAM is worth the extra bucks.
My system including operating system cost about AU
$2,000 (about US$1,600), and would be looking at about AU
$1,000-AU$1,200 more (US$800-US$1,000) if I had have gone the faster CPU, GPU and 4GB of RAM. Based on what I have seen so far the big-end system is going to give reasonable performance, but not sure if it is worth the $$$ right now considering it still ain't gonna give you 20+ FPS with all the sliders to the right with the current version of FSX.
My recommendation would be, based on very recent experiece, if you can't afford the best available then wait until FSX goes full DX10 and spec your hardware then.