Im pretty sure all the dualies can run with just a single processor installed. I'm running dual mp 2800's and i have no intention of upgrading for a while. Duals are definately the way to go and if you can afford it opterons are probably best i think.

My machine totally kicked the butt of an Athlon 64 3200 with loads of memory and scsi raid so even the dual AMD's, especially with thier cheaper costs right now are a great upgrade for anyone using 3d software.

As for memory, well that really depends on the board you're looking to purchase.