The other problem is arguements between directX9 and directX10 as to who manages the virtual addressing for video memory. On DX9 applications this is done by the application, on DX10 this is done by the OS. Run a DX9 application on a DX10 OS and you can get double the amount of virtual address space used. This is a common cause of failure to run DX9 games such as The Witcher and Test Drive Unlimited under Vista. MS released a hotfix to work around this problem and if you've got a gfx card with a large amount of ram I really recommend using it:
Virtual address space usage in Windows game development