Vray demo'ed a GPU based real time rendering system at Siggraph. It looks like it is running parrallel to 3DS in the demo. So it should be easy for them to eventually do a plug-in for Revit.

Rendering is 20x faster than a 8 core CPU. And they get real time display at 7-12 FPS. Very, very impressive....

http://www.cgarchitect.com/news/SIGG...ROUP-GPU.shtml

They are using a pretty modest Nvidia card ($250-$300 range/240 GPU cores.). The thing should fly if you hook it to one of the Nvidia Tesla systems (960 cores!).