UT2007 will most likely support DirectX 10, but not require it. Most games coming out next year will be the same way. A single X1900XT is going to last you a long, long time assuming you only want to play with DirectX 9 settings. The capabilities of DirectX 9 have been mostly used to thier potential, which is why we need DirectX 10. (and because Bill Gates needs more money)
To show you the difference between the 2, here's a comparison:
http://www.istartedsomething.com/20060817/directx10-fabulous/
Edit: Looks like UT2007 will NOT support DirectX 10 according to this article
http://en.wikipedia.org/wiki/Unreal_Tournament_2007