Adding a new processor I've got a Mac Pro (Jan-2008) 2.8GHz Quad-core Xeon with one processor. Do all the Mac Pro's have the two sockets motherboards even if it's purchased with one processor? If so can I purchase an equivalent Intel Xeon proc and install it to have the 8-core?
No, single processor Mac Pros ship with a single socket motherboard, and use Xeon 3000 series processors, not the more expensive Xeon 5000 series needed for dual socket support.