Check out Mach Studio Pro!
So what is Mach Studio Pro? William mentioned it in his blog recently and Steve Bennett mentioned it in his blog in May 2009 after getting to see it and again in November 2009, so you can read their thoughts too. Here's a description from the Studio GPU site.
"Working at the Speed of Thought"
"Real-time interactive workflow and rendering has always been the holy grail of the CGI industry. But crossing this boundary has proven elusive even to those with the highest-end multi processor environments .... Until now.
Here's an image capture of one of several sample videos you can watch.
If you try out or buy the product and want to use it with a Revit model you can download the exporter which will install and provide the export functions within Revit. You can check out the download page HERE.
If you want to check out the tutorial that is available for it then you can download that HERE.
It was very impressive seeing rendering results as soon as I made any change, immediately, no waiting. Revit models do suffer a bit from performance issues similar to Autodesk's Max environment but some careful model exporting can provide a model you can set a scene for quickly and then test out everything else. When you are ready for a final it still takes a fraction of the time for the final than anything else I'm aware of.
Part of the price is the included high end ATI 8750 Video Graphics card.
I've been quietly waiting for a chance to write about this. Last fall I got a chance to see this product up close and work with them to support Revit more directly. They released the exporter for Revit really quietly, so quiet I missed it too.
Time to check them out!