Not noticed any real difference when i upgraded from XP to Vista 64 recently.
The main help Vista could be is that you could whack in loads more memory and Vista could use it where XP can't and FSX loves memory, thats' why I went to 4GB.
As for the DX10, the enhancements for that should be coming out at the end of the month as part of the Acceleration expansion pack, to be closely followed by Service Pack 2.
Taff