Quote from http://www.starcraft2.com/faq.xml:
Q: Will StarCraft II take advantage of DirectX 10? What other graphical goodies are included?

A: The game will be compatible with DirectX 10, and we're still considering whether there will be exclusive DirectX 10 graphic effects, but the graphics engine will also be very scalable to ensure that a wide range of different systems will be capable of running StarCraft II. The new engine is also capable of rendering very large units, as well as large numbers of units on screen together. Havok physics have been integrated into the engine for added realism as well.


I think Direct9c X ... ups... DirectX 9c would be the minimum requirement.