OK, we need to get some things straight with AMD's 64bit line of processors. The difference between 754 and 939 is not the die size, or just the number of pins they use. They are not the exact same core, and believe it or not, the speed ratings do differ.
Socket 754 is essientally the same core as the 939, except, that it only has a single channel memory controller. That is basically the only difference, but like anything, when something is removed or changed..there are going to be variances. It started on a 0.13micron manufacturing process, but it is now on a newcastle core that is 90nm. The production of processors will inevitably end, but only after the release of Sempron processors on the 754 line. These are basically Athlon XP processors changed to work on the 754 platform, and of course having their own single channel memory controller. They are NOT 64bit.
Socket 939 on the other hand is the base core just with a dual channel memory controller. It too has gone from 0.13micron to 90nm. The 939 line will be AMD's major platform and will recieve almost all updates including dual core processors.
Here is a link to more detailed information about them and also some benchmarks showing the speed difference between 754 processors and 939.
Also, the 940 platform is for opterons, and they will probably be more stable, since they are a server platform, but I wouldn't necessarily recommend them for home use, since they still require registered ram. Also, the cores are different there as well.
