Blazkowicz
Legend
Dreamcast's PowerVR2DC aka CLX2 graphics chip was a custom variant of PowerVR Series 2, it had 32x32 tile size (i think that's the term) than the Neon250 aka PMX1 PC version which was 16x32.
I believe the GPU has to be custom for at least one reason : include a realtime, low latency hardware video encoder. (well, blocks of still images with some high end compression)
one is sold on windows servers for the same use :
http://msdn.microsoft.com/en-us/library/gg607502(v=vs.85).aspx
it could be a separate chip but feels better at home on the GPU. and maybe they use hypertransport or PCIe 3.0 or something.