So off the top of my head:
- For HyperV & Xen you need VT-enabled hardware, latest Intel & AMD only
- Go for more than 4gb of RAM, part of this is about pushing a few limits, and you'll want to run a few VM's at the same time
- Get a hardware RAID card - I think the LSI MegaRAID SATA-150 is about as standardized and supported as it gets - ESX/ESXi work fine, and Xen and HyperV should all work
- Use laptop drives for your RAID set - they fit much nicer into cases (4x320's gives you 900gb)
- Go for 2-3 network adapters - Intel or Broadcom only (10/100 is fine)
Pick a base OS and run VMWare Server - trust me on this. Instead of reinstalling the OS off of cd, you're mounting the cd in the VM and doing your installs without the legwork. You can also download pre-built demo appliances so you spend your time dealing with the product (Oracle, IBM, etc), rather than tweaking out your CentOS config.
If you're going to stick with VMWare ESX/ESXi you can get any server hardware from the last 3 years. Sun x4100's, Dell 1750/2650's, HP DL380/385's all work fine, though RAM is mostly still expensive
Someday somebody has got to decide whether the typewriter is the machine, or the person who operates it.