Xeon or AMD equivalent processors are best because they have a lot of second and third level cache.
Two gigabyte network card are required, but 4 are suggested. One network interface for traffic and the other one for migration or administation.
Hard discs faster as you can permit (money question): SCSI, RAID configuration.
Memory depends on how many virtual machine you are going to handle (operating system, jobs to carry out, ...). You have to remember you have to attribute some memory to host operating system.
If you plan to use VMware ESX (or free version ESXi), you have to chose a compatible server, because drivers cannot be available for a generic own build server.