Is not wise to go to DirectX 10 now.
Maybe after a stable O.S. and a official DirectX 10 SDK release.
If you wish you can run current Java3D DirectX 9 on MS Vista.
Like Vista OS, DirectX 10 is too much new, and will change a lot until be
stable enough for commercial application. There is new DirectX SDK each two
months, and sometimes not so compatible with previous ones.