Looking on ebay, I found a seller selling the logic board for Dual 2ghz CPU $115, and the matching CPU's 2ghz w/heatsinks for $58 (each). This seems like a decent upgrade from the single 1.6 cpu, although before I pull the trigger I just want to make sure the PSU will work? since apple keeps things locked down.
I think I will also need to find the dual cpu ait duct with fans, since my 1.6 only has the single fan.