I was wrong about XP needing to map all 3 GB of video RAM.
It is more accurate to say that Windows XP limits the GPU to a 256 MB window. The GPU would perform a lot better if all of its memory was mapped for direct access.
The newer pieces of GPU hardware and drivers are using zero-copy direct memory access with addressing that is the same on the GPU and the CPU which allows sharing of data structures without copies or modifications. That can't be done in a 256 MB window, or if it can, not well.