Well, the Opteron is the AMD processor targeted to the server market. In all Opteron CPUs the memory controller works in dual channel mode so as long as you have two memory modules or an even number of memory modules are used, you have dual channel capability. Socket 940 requires ECC memory modules too so there's another difference between Opteron supported PC and Athlon PCs.
The Opteron has another big difference when compared to the Athlons and that is the number of supported HyperTransport busses. All other CPUs using Athlon 64 architecture and Opteron 1xx series support only one HyperTransport bus. Opteron 2xx series supports two HyperTransport busses while Opteron 8xx series supports three HyperTransport busses.