[mindslight]

For a rough first pass, I would just look at monetary cost.

I was going to dispute OP's comment. Thinking of my old Athlon server that used around $20 of electricity per month, cost about $600 new IIRC (mobo+CPU+RAM), and ran for more than a decade ($2400). But there has been a concerted effort to lower device power consumption, and my current 5700G server (two generations later) uses only around $5/mo of electricity.