Once you apply the service pack to FSX, your rig should run very well. Received wisdom is that 20FPS is the minimum before you run into stutter (although I personally feel happier with 30FPS). Your rig should get you above 20FPS in 90% of situations with scenery and other display options in the mid range - which will still look reasonably good). The 10% where you and even people with Q6850 / 8800 Ultra rigs will get stutters will be flying around major airports (eg JFK and Heathrow) with display options in the high area. Something we learn to live with.
I personally only fly light aircraft and never go to those places

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However, your rig will run FS9 (FS2004) like a dream. You'll be in the 30-60 FPS range with settings maxed.
FSX is really just a pretty, slightly commercialised version of FS9. Off the top of my head, (this list is what I have noticed without going through a quality check)you get:
* Missions - the best bit IMHO - where you get a tricky task ranging from red bull air racing (totally hookable) to rescuing people in jungles.
* better scenery graphics - slightly better trees, much better water, clouds haven't really moved on IMO
* different and improved stock aircraft. Actually the ones in FS9 which have been dropped were fun.
* marginal improvement in aircraft graphics. The quality 3rd party aircraft are far better. IMO - the light aircraft of choice is the Realair Siai Marchetti, which blows away anything produced by MS. They have versions for FS9 and FSX. The FSX version is marginally better.
Apart from FSXs prettier interface, I believe that FS9 still represents excellent value for money. There is a massive freeware market out there, and on mocern machines - the sim runs very, very well. If I was new to this - I would get that over FSX, save some money, decide if I like it and upgrade when the code is tighter, and works well with DX10 and Vista.