The 64-bit CPU, system and OS are considered as a major new generation in hardwares, softwares, system technology (chipset, memory bus, HT, ...) and silicon technology (90 nm SOI), ....
I prefer the 939 platform for its
- 128-bit memory bus, twice the max memory bandwidth compared to 754, estimated faster than P4 dual channel QDR
- plus on-chip dual channel controller
- 939 allows use of non-registered, non-ECC memory (940 though has 128-bit memory bus and dual channel controller, but requires registered memory)
- future hardware compatibility (130 nm CPU then to 90 nm)
- upgrade ability and reusability of older 939 motherboard/CPU in the future if needed
- 940 motherboards are considered more costly to make than the 939 counterpart due to the pin layout of the 940 CPU
AMD have three CPU socket types for 64-bit CPU, 754, 940 and 939. The difference between them is not simply a matter of pin count, there are implications for each on performance, cost for components (CPU, motherboard, memory, ...), and upgrade path. IMO, I would only get the 939 CPU and motherboard (current plan).
Even cannot wait much further, I would get a first generation 939 CPU (hopefully a reasonable priced 130 nm CPU) and 939 motherboard, since 939 CPU's and motherboards would be reusable with latest hardwares down the road (e.g. next year).
This 939 platform memory bandwidth, as estimated from some test data (so result is preliminary), is impressive. Its efficiency is around 86-90%, which is 15-20% (to be confirmed with more 939 test data) better than the P4 QDR dual channel counterpart.
Its effective bandwidth (not max), running at the same memory bus speed, is about 15-20% higher than that of P4 QDR dual channel and 81-89% higher than that of 754 platform or nforce2 dual channel.
Estimation and importance of 939 platform memory bandwidth (page 19)