1) "OK" is definately a matter of opinion with FSX..
Your sys will run FSX "OK" but, not great.
Fact is, there is no sys currently avalible that can run FSX anywhere near its full potential.
So, you have to taylor FSX to a given sys....... dumb down the areas that are not to important to you at present to get acceptable performance. This is my opinion of course.

2) VISTA alone will probably reduce performance a bit by most accounts.
However, VISTA coupled with a DX10 card will give a performance boost. Just how much improvement that combo will allow, no one can say for shure at this point. It is "wait and see" for all of us.
3) Yeah, the missions can actually be fun at times.
4) Sadly, no
As Hagar suggested , try the demo to get a general idea of what FSX has to offer and how it wiil run on yor sys.