X86-secret speculates (based on several sources) that AMD will launch his Socket 939 processors on the 29th of March. Exactly the day that Intel is going to launch its Grantsdale chipset and Socket T (LGA775).
According to this French site we can expect the launch of the following Socket 939 processors from AMD on the 29th March: the Athlon 64 3400+ and Athlon 64 3700+. The Athlon 64 FX 53 will normally appear a little later and there will be no Socket 939 versions of the Athlon 64 3200+ and 3000+ according to X86-Secret
In a nutshell Socket 939 will bring Dual Channel Unbuffered memory to the Athlon 64. At the moment the Socket 754 Athlon 64 can only work in single channel memory configuration.