Look for a FSB of 800 Mhz. This is the current max as far as I know for PCs. The best would be a board that allow dual Opteron, just get one now (the cheapest) and after xmast, get rid of the CPU and replace it with 2 dual core Opteron. This will give you the equivalent of 4 CPU in a single machine!

This is the most #future proof# solution I can give you.