I would like to build a system running VmWare ESXi hosting 4-6 virtual machines. Do I need a dual processor mobo? Should I go with 1366/i7 since they have hyperthreading? If I get a dual processor mobo do I have to fill both slots or can I start with just one CPU?
Did you visit communities.vmware.com? Most desktop motherboards require additional Intel or Broadcom NICs, the on-board RAID doesn't work (a supported RAID controller might be required), etc. It's best to use a server board that's been successfully tested with ESXi 4.0.
Will your VMs be mostly idle? If so, then a Quad Core CPU is fine. I run a few VMs and the bottleneck rarely is the CPU or memory. For me, hard disks are the main bottleneck.