Well it has an upgraded DX9, they call it DX10, but I don't think it is the final version.
And yes there are a few problems with some programs, which is my main reason for using it on it's own on a spare computer. The increase in fps can be quite a lot some of the time, I find the best performance is without any firewall, trojan, or virus checkers running, or similar. So my spare comp. is not connected to the 'net. Only for very short sessions, for activation, etc. Then I pull the plug.
Do you realise the size of download for Vista, over 2.5Gig. I'm on DSL and it took over ten hours. Luckily, I used my spare comp. before I had used it for the Vista install.
In all honesty, unless you are well up on computers, etc. I would wait, FSX will run quite well on XP, if the settings are kept down a bit. And you have a pretty good computer, specially if you are adding some memory.
If you take your time, some of the problems which are present now will get sorted, and make it much easier for users to install with less trouble.
These are my feelings on the subject, maybe some others here will give their views.
One other item, which may influence you final decision. FS9 does not like Vista. So you would only have FSX. If you want both, you should stay with XP.