Opteron 275 and 875 look alike and the 800 series for quad- and eight-CPU configurations will also run in dual-processor systems (which is what we did).
The dual-core Opterons based on the Italy core are technically very related to the Athlon 64 X2 Manchester core, as both have two 512 kB L2 caches for each of the cores. An Opteron derivative of the Toledo core (with two 1 MB L2 caches) is not yet available.
Patrick Schmid was the editor-in-chief for Tom's Hardware from 2005 to 2006. He wrote numerous articles on a wide range of hardware topics, including storage, CPUs, and system builds.