If the game even has DX 10 capability and you have a DX 10 video card you can still choose to use DX 10 it or not to use it. Or so it has been for me so far.
Vista is backwards compatible. It works with most all DX 9 games and with DX 9 video cards. DX 10 is very optional.
Just what I was hoping, thanks a lot.
Looks like I'll be using Vista with my new rig.