You know DirectX is cumulative, right? So you could run a DirectX 6 game on your DirectX 7 card through DirectX 10?
DirectX10 contains all the technology, APIs etc that DirectX 9 has. If your card is only DX9, then it will only use the DX9 calls obviously.
The lower performance is because you're on Vista, not because you're using DX10. Basically, you think you know what you want but you're wrong.
My recommendation is again to upgrade to Windows XP.