CPU: 64bit (Intel EMT64 or AMD64)
Intel VT/AMD-V capable CPU/Mainboard (for KVM Full Virtualization support)
8 GB RAM is good, more is better (grab as much as possible)
Hardware RAID with batteries protected write cache (BBU) or flash protection
Fast hard drives, best results with 15k rpm SAS, Raid10
Two or more Gbit NIC (for bonding), additional NICīs depending on the preferred storage technology and cluster setup
What exactly you get depends on your budget and where you want to get it from.