1.) Well, it will take another while 'till 939 becomes available...
But, if you want to have a reliable upgrade-path, buy a single or dual Opteron board. It's not that expensive, but AMD will keep socket 940 for the Opteron, and a dual-core socket 940 Opteron will become available next year - using the same chipsets (ie, buy a MSI Master FAR2, Gigabyte or Tyan K8W board with a single Opteron, upgrade to two Opterons if you need more power, and you can still upgrade to dual-core Opterons next year (for 4 CPU's)).
The Opteron has the most efficient multiprocessor design available today, each CPU increases the performance by 70 to 95 percent (not 30-70 percent like the AthlonMP or P4 Xeon. And dual-core != HyperThreading).
2. Yes, it will become available sometime this year, and was designed for AMD64.
3. Don't know about that. But if you really want a reliable system, go Opteron - if it's good enough for mission-critical servers, it should be well suited for a DAW...
4. If you want to use SATA, get a true, native SATA HDD. (It's all SCSI for me...

)
Hope this helps...