Paul,
I’m not absolutely sure, but I believe if you’re talking about dual-processors, you’ll have to go with a new Xeon setup. The i7 chips are mostly the same as newer Xeons, but don’t have the CPU-to-CPU communications to enable use on a multi-processor system board.
But unless you are doing a lot of truly multi-threaded rendering, you might be better with the fastest single i7 chip you can find. I recently added 2 new dual-Xeon (5500) machines built on a SuperMicro 1U rack server platform, and they don’t do all that much compared to my older (2 years) dual-Xeons, UNTIL I’m doing Mental Ray rendering, which maxes out all 16 cores per machine. Then they are quick like bunnies!
-Jeff