i believe tyan offers motherboards that support 2+ cpu sockets, each with 2+ cores per socket... so, you effectively have a minimum of 4 individual cpu cores then... i couldnt tell you specifically which boards they offer, and performance reviews on them, but if you go to http://www.tyan.com/ , you should be able to find out some product info, and search from there
So from an intitial scan of everything it looks like the Intel 5000 series chipset is what is also used in the Mac Pro systems with socket 771 dual core Xeon CPUs. The main benefit being that the 5000 series has the higher 1333MHz FSB than other chipsets.
I guess the next problem is determining which chipset variation (P, X, V) is the better option. Using Intel's comparison chart says that all specs for P,X, and V are identical. Might have to do more digging to find the differences. However the Intel motherboards single out X series for the workstation category. I just don't have a clue to why yet. I guess I need a THG on that. :-)
Then step two will be sorting through all the motherboards to determing which ones will be best for an HD video/audio workstation. And to determine if another chipset/motherboard would perform better than the Intel 5000 series cipset/boards.