The Xeon 5500 series processors supports both registered (RDIMM) and unbuffered (UDIMM) DDR3 memory at speeds 800, 1066 and 1333.
So far best post on memory:
The previous generations support only fully buffered memory. So using unsupported memory on the platform like fully buffered on the new platform 5500 series or unbuffered memory on the previous generation wont even work.
All the best,