Hardware

Why not sell (some or all) those old units off. Build a new computer with a 4 or 6-core processor, lots of RAM, lots of hard drive space, multiple LAN cards and create multiple virtual machines?

Or if the workstations are powerful enough, you could probably set up virtualization in 5 machines with each machine running 1 real and 1 virtual machine. That's half the hardware but you still get 10 servers.

I can think of plenty of things to do with that many servers. Join a distributed computing project for instance. It will make the electricity bill go up, so it will either way defeat the purpose.
Investing in ssd's is hardly worth it, given that they cost more than your electricity bill for a year or two.
If all else fails, place the servers right under the window silk and see if you can reuse the heat coming off the energy supply and lower the heater setting.