One of the really great things about vmware ESXi is that it’s very happy to support older hardware.
Microsoft Hyper-V and Citrix XenServer insist that you run only the very latest hardware – amd64/em64t and hardware virtualization. Which isn’t completely unreasonable, really. But ESXi supports older boxes, with boring old i386 processors.
Sure, you can’t fit as many virtual machines on these older machines as you can on a modern box with hardware virtualization, but you can pick up older machines with shit-hot disk subsystems for peanuts on ebay. They make great build machines, for example, since cpu speed matters a lot less than disk speed for building, and you can stick a couple of VMs on (say) a Dell 2650 or something. (These older machines are probably also pretty good as nfs/iscsi boxes too, but I’m not doing that right now.)
In fact, the only draw back of ESXi’s inclusive approach to cpu support is that it’ll run on a modern machine that you forgot to enable hardware virtualization on – like pretty much anything from Dell! I keep being bitten by this!
