Having used FSX for @2.5years now, I can say it sure is a strange beast, not like any other game or simulator out there. FSX is indeed mostly depended on CPU, due to both its age and that there are no more service packs beyond SP2 (company who did FS is now sacked by MS I believe) so FSX is not exactly optimized for today's hardware. It does not fully utilize multi-core, hence why a dual core 3Ghz will run just as fast in FSX as a quad core 3Ghz. Also its doesn't use memory very well, I believe FSX can only use a max of 3.5GB so its 32bit in nature, so having more then 4GB for just FSX is of no use. When it comes to GFX cards there are 2 choices, ATI or Nvidia, both have pros and cons: ATI does indeed have trouble with the cloud shaders and has a hard time with large volumes of clouds on the screen, especially when flying through them, so the slower the CPU and the older the ATI card is, the more you will suffer with the clouds. ATI does do 2 things better in FSX then nvidia cards, 1) It can handle FSX in full screen without the regular CTD whereas nvidia cards FSX users always run FSX in full window version as CTD seems regular with nvidia 2) IMO ATI has better MIP mapping and that shows up real well in FSX when you looking at distant object and distant scenery (photoreal scenery & LOD radius settings).
I would highly recommend you carefully read this link
http://www.simforums.com/forums/forum_posts.asp?TID=29041 not only has it many very good tips on how to tweak your FSX but also states system/PC/GFX card setup for FSX, btw they also mention in there that after tests, the ATI HD5870 runs equal and even better in some cases then a nvidia GTX285.
In the end I can suggest a few things 1) Get as much horsepower out of your CPU cause FSX loves using it up 2) you can make a 'huge' difference in both looks and performance in FSX with careful software tweaking so read the link above 3) FSX at default looks pretty poor compared to addons both payware and freeware, I highly recommend REX2.0 for starters, look at some decent aircraft addons (I recommend the UHDT range from McPhat for both the Super 80 Pro & CS 757) and look at scenery addons (I personally like the look of properly made photoreal scenery check out Earth Simulations) 4) whatever GFX card you are getting I suggest something with more then 512MB, as textures addons in FSX are getting better but in return use up more resources. If you are going for ATI, I suggest the HD5870 but for better value stick with a nvidia GTX260.
Good luck.
PS If you want to see some screenshots of what my FSX looks like, simply check my recent posts and look at my posts in the screenshot thread. I run it with a E8600 at 4Ghz and an ATI 4870x2 2048MB so it will give you an idea of what FSX can look like.
Here is just a recent one I made:
