Doesn't matter whether you use Xen, Vmware ESXi, VMware Server, or whatever. Virtualization is how your customers will deploy your product, a way you can reduce costs, speed product cycles, and as a PM you should learn as much about it as you can.
So buy a well-specced out box, with 2-4 CPU cores, 4G - 16G of memory, several sata drives, and install a 64-bit linux with the virtualization solution of your choice.
From that point, create VMs as you need. You can even turn your existing machines into VMs with VMware converter.
As other posters have pointed out, you can donate or recycle the old hardware.
Good luck, and have fun!