1, I would definitely recommend FSX over FS9. However, you should consider getting either Lockheed Martin's Prepar3d (which is essentially FSX with better graphics and bug fixes), FSX: Steam Edition (which is a bug-fixed version of boxed FSX) or X-Plane 10 (which I'd recommend least out of all of them if you're going for a truly immersive, in depth experience).
2, 4GB of ram would definitely be enough to run any sim on fairly low settings.
3, The base product is fairly realistic; if you're willing to spend some money to get a good payware addon (A2A and PMDG produce AMAZING products) it can be unbelievably realistic.
4, For me, at least, it's very fun.
5, In FSX you can get a large number of freeware and payware planes.
6, No idea.
7, No idea.
8, Boxed FSX no longer has multiplayer, but FSX: Steam Edition has multiplayer.
9, I think there are around 25 or 30 thousand. I might be totally wrong though.
10, The more addons you get, the more indepth it can be. For example, it can take weeks or even months to master flying a PMDG aircraft based on your previous level of knowledge.
11, That's your choice.