The Dreamcast uses a special WindowsCE kernel with DirectX 5. Yes, 5!.
It uses a n'force version. Yes, like a GF4, but with its level of syncronization, can be better that some configurations of PCs (If you have a P4 1.3+ Ghz, RDRAM PC600, UltraATA100, DVD 16X, the X-Box is a BABY!!)