Brent from [H]ard|OCP / [H]Enthusiast has written an article regarding DirectX 10 and how it will impact gamers next year when Vista finally ships. There are some major changes coming with DX10, and a whole new generation of GPUs are on the way that are required for full support. For a taste of what's to come, check out what Brent has to say.
"DirectX 10 will be available to Windows Vista users only at its introduction. You will not find DirectX 10 being released for the Windows XP operating system. DirectX 10 is deeply embedded into Windows Vista operation and we currently know of no plans by Microsoft to allow Windows XP to officially support the new API. Also embedded into Windows Vista is DirectX 9.0L to allow for compatibility with DirectX 9 components. Think of it like two separate DirectX systems. We will have DirectX 9.0L for DirectX 9 hardware and we will have DirectX 10 for DirectX 10 capable hardware. If you want DirectX 10 you will have to go with Windows Vista as your OS. Because of this we will see an expensive upgrade path associated with the experience of DirectX 10. You will need Windows Vista, DirectX 10 hardware and of course some DirectX 10 coded games."