I can almost guarantee you that DX10 won't be available for XP. XP won't die as soon as Win7 comes out, though; they provide a "Windows XP Mode" for the Professional and Ultimate editions. You can download Windows Virtual PC and run a free, licensed copy of XP in it.
Slightly on topic protip: The newest virtual PC requires virtualization support from your processor. If you don't have that, you can just use VPC 2007. You'll have to do a bit of hackery to get the virtual driver to work, but it is possible. Then you just make a new virtual machine and use "Windows XP Mode"'s virtual hard drive. It works just fine.