FS9 will work well with pretty much any mid-range machine on the market. If FS9 (or even FSX with low/med settings) is your goal, stay away from those huge markups on Alienware and the top of the line XPS systems.
Big issues from my experience:
CPU: Obviously faster is better, I believe Intel still has a slight performance lead but neither manufacturer will let you down. Multi-core is about all you can get now--and FSX can use quad-core processors (though it tends to put more load on the first two in my experience). I don't know about FS9.
RAM: More is better--especially if you are running VISTA. Keep in mind if you are using Vista or XP
x86, there is no real point in getting more than 3gb of ram--Vista/XP x64 based systems may see a gain with 4gb or more. On my system, Vista generally eats up about 1 gig itself, and FSX will take another 1.5, Keep in mind, these are rough averages--both sometimes use more. By the way, if I had to do it over again, I would have invested in higher quality RAM rather than just lots of RAM. I don't have much experience with it, but I think better timings could boost my performance.
Graphics: NVIDIA 8800 series reigns supreme at this time. I personally have not tried FSX with duel GPUs, but based on what I've read I decided to invest in a single high-end GPU. Some of the new 8800 models (the GS
I think?) are have only marginal performance decreases when compared to the top of the line GTX and ULTRA, but cost a fraction as much. Great place to save some money if you don't need every ounce of performance!
To get an FS
optimized system at a reasonable price, I'd suggest building it yourself or looking for a firm that builds highly customized machines. If you have the cash, the big Dell XPSs are some of the best on the market. If you are willing to take a bit of performance hit on FSX (still good with FS9), look at a midrange model from Dell or another manufacturer--more convenient than building it yourself and the price for what you get is much more reasonable.
Anyways, my 2 cents....