FSX runs on Vista ok here. The DX10 offering is only available on FSX SP2 and is a watered down DX10 preview (much more was promised at launch time). It is available as one of the settings in the FSX control panel. If you don’t select the preview mode it will run under DX9 in Vista, the same as on XP.
Some people say that running it normally (DX9) in Vista gives better framer rates that when running under DX10 preview mode, others say the opposite. Running FSX in XP appears to give comparable frame rates to running it in Vista in DX9.
I believe that FSX is more hardware dependant the OS dependant, yours looks ok.
Given that the Nvidia Vista drivers are now mature and that Vista SP1 is nearly upon us I would stick with a Vista build.