DX8 is when I finally said "Wow, this is really good!". They finally ripped out all the DDraw crap and made several important additions to the API. DX9 is even better. DX10 is shaping up to be really good; much more orthogonality, and stream out has some cool potential. I'm still kinda meh on their geometry shader implementation however. Anyway (I'm rambling), you could probably drop down to DX7 and still be pretty happy, although IIRC every dist of XP has DX8 installed by default. My last project was DX8, and no one reported any DirectX problems (caveat: we retained a software rendering mode option... I have no idea how many people are using it). And switching from OpenGL should be pretty easy for you... Probably easier than switching to GL from DX (many operations are more explicit, there are less "crap, what texture is bound right now?" situations).