well, first of all...there are three versions of the opteron family....
1xx, which is for single-processor environments
2xx. which are for dual-cpu configs
8xx, which can be 4 -8 cpu configs.
the fastest 2xx opteron is the 242. it has a 2.6Ghz clock speed and 1M of cache. sounds a bit like the athlon64 FX-55, right? well, it mostly is, but the opterons can only use ecc registered ram, which is a bit slower. as well, opterons have multiple hypertransport links, which the FX's don't
AT did an article on the opterons, go to the homepage, then IT computing.
Cheers!