core wise, absolutely no difference. the X2's with 1MB cache per core and the opterons are exactly the same core, namely Toledo. but, the opterons are made for servers and workstations, so stability is a higher issue. so opterons are usually a higher grade sillicon than the X2's, and thus, OC better/higher.
when a CPU gets tested, and its a real good one, its branded a Opteron.
if it doesnt come through the strict testing process, but its still able to run at high speeds, its a X2.