I have a S5520SC workstation motherboard with a single X5570 Xeon processor. I wish to add a second processor and wondered if I can install a X5670 Xeon (hexa core instead of quad core)? Is it possible even if the processors are not exactly the same?
No you can't mix a Xeon X5570 with X5670. Mixing processor is strictly limited to same family, same frequency, same cache size, and same # of cores. Normally +/- one stepping is allowed, but the CPU with lower stepping should be installed in socket #1.