FSX works fine on Win7Pro64. I use Dx9 mode, but thats mainly because many aftermarket aircraft have graphic anomalies in Dx10 Preview. IMO DX10 doesn't really add much anyway.
How is the selection of aircrafts in FSX?
I think the largest difference between FS9 and FSX come with the cockpits. If you still prefer 2d panels, the FS9 is fine. If you've made or are willing to make the jump to using 3d cockpits full time, then FSX offers a much better experience. Otherwise the games are very similar other than the fps sucking graphics engine in FSX. The latest FSX payware planes are also extremely well done for the most part.
If your willing to buy some payware scenery and terrain packs for FS9, you can get it very close to FSX in overall graphics quality, without the pain.
So only commercial aircraft available in FSX?
http://www.x-plane.com/ isn't half bad if you want a realistic simulator.
X-plane is more of a core level flight simulator, in that it excels at airfoil simulation. What it lacks is everything else. Although it can brag about its global topographic coverage, the airplane cockpits suck in general and there is virtually no scenery included in the box other than autogen (which in my opinion is superior to FSX Vanilla) not to mention being complete anemic of ATC and flight planning. Comparing FSX to X-plane is like comparing Windows to Linux. X-plane ultimately has more potential but a lack of serious community support as well as a less than adequate out-of-the-box experience ultimately keeps people coming back to MSFS.
But Microsoft shutdown ACES Studio, so their flight sim is a dead end. So unless someone else comes along, the future is X-Plane.