When compared to an Opteron 248, te 866 was a winner in all tests - PC Mark, Aqaumark, 3Dmark 03, Super Pi, Content Creation and Business Winstone and Sandra. The differenes varied from 1% to more than 100% in Sandra, showing that some benchmarks were not ready for dual core let alone dual processors. More interestingly, even a dual Opteron 248 were not able to beat a single Opteron 866. AMD has done a fantastic job here.
Even though, there has not been any straight comparison yet with any Intel processors, you might find the GamePC's review comparing the 248 and the Xeon 3.2A interesting and of course, Anand's own part I and part II articles comparing Intel Dual core performance with one AMD Athlon 64 3500+.
Now what we need is some comparison between the Athlon 3500+, the Pentium IV 2.2Ghz and the Opteron 866. Then let's try using the Opteron 866 vs the Opteron 852 and the latest Xeon and Pentium D 2.8Ghz plus some professional benchmarks - Video encoding, Linux etc. Mind you, AMD is launching its Server parts first while Intel has released the desktop Pentium D 2.8Ghz for benchmark. Now may the fun begin. µ