Recommended

• Dual or Quad Socket Server (Quad/Six/Hexa Core CPUs´)
• 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 Gbit NIC (for bonding), additional NIC´s depending on the preferred storage technology and cluster setup
• Fencing hardware (only needed for HA)

Minimum (for testing)

• CPU: 64bit (Intel EMT64 or AMD64)
• Intel VT/AMD-V capable CPU/Mainboard (for KVM Full Virtualization support)
• Minimum 1 GB RAM
• Hard drive
• One NIC

Testing Proxmox VE as a VM

Proxmox VE can be installed as a guest on all common used desktop virtualization solutions. But note, KVM will not work in almost all cases - only if your software allows nested virtualization (experimental feature).

OpenVZ containers works well.