got a response back from autodesk (was able to finally find somewhere to actually get in touch with them for free)

Thank you for contacting Autodesk.
In the 2010 edition, there was a limitation of four cores. However, in the new 2011 edition, there is not a limitation to the number of cores that Revit can use while rendering.

hmm....pity the board I've been looking at will be ~700 as the older tyan board has been retired