Aiming at Direct X 10 I think is the wrong choice. The first reason is that not many people jumped to Vista. On the other hand, if you release at the time Windows 7 sits on store shelfs, then you've got something going. But, Direct X 9 is obviously a good choice. I'd look at the differences between Direct X 9 and Direct X 10 to sort out the elements that you agree to work with. And, how much time do you have on your hands to write such a huge project?