It's transitional for the moment which is what is causing the confusion
Socket 940 requires registered memory. The chip that will run on socket 940 is & will continue to be the Opteron processor. Available in up to 8 way configurations, the opteron is designed for VERY high end workstations (video editing / CAD) and servers. Initial shipments of the "FX" family of high performance processors used socket 940. These have now been migrated to the socket 939 pin product.
Socket 939 is AMD's "high end" processor family. It does not require registered memory and can utilise dual channel memory (ala nForce2 & Athlon XP in it's hayday). 939 is effectively the replacement for the Athlon XP family of processors. Gamers, enthuiasts & so on would head towards those processors
Socket 754 was the first iteration of desktop processors from the Athlon 64. With the move to socket 939, socket 754 will exist. However it will limited to a range of slower, less expensive processors which are the equivilent grade to the Duron processor. Most Sempron (a cut down Athlon 64) will use socket 754. At the moment, 754 is slowly transitioning to socket 939.. which is why there is combinations of both. AMD will release a 32bit version of their processor (rather than the existing 64bit) and it will use socket 754. Cheap too.
Socket 940 = Servers, workstations :: Replacement for Athlon MP mulitprocessors
Socket 939 = Desktops, enthuiasts, power users :: Replacement for Athlon XP
Socket 754 = Cheaper processors, less performance :: Replacement for Duron
While socket 754 may be the currently most popular Athlon 64, eventually the transition to Socket 939 will be complete and all three families of processors will happily co-exist and meet a requirement depending on budget, performance, 32bit or 64bit requirement & target usage needed
The FX family of processors will always have the edge over a standard socket 939 Athlon 64 and this would be the equivelent of a Pentium 4 Extreme Edition.. but without such a high price tag (well until Intel reduce their price, which they no doubt will).
Hope that clarifies it...
