Key to understanding this is the massive 1944pin socket used by the 6100 series. This is needed due to the fact that the processors essentially consist of two CPUs sitting together on the same package. These CPUs communicate with each other using HyperTransport 3.0 links, which have been tweaked from previous generations to improve communication between CPUs.
In less relentlessly techie terms this means that the Opteron 6100 series is faster and cheaper than before. In fact, price performance is the mantra being used by AMD for this launch.
A big part of this is the removal of what it calls the '4p tax' thanks to the introduction of new HyperTransport links inside the CPUs. By this it means that all of these new CPUs will work in four processor systems - historically these kind of CPUs have cost more than dual processor ones.
With Intel due to launch its Nehalem Ex based Xeon processors soon the server processor market is set to heat up.
These new Opterons will be coming in servers from HP, Acer, Dell, Cray, SGI and others.