Ask HN: Best Plug Computer or energy efficient server?

I am looking for a high performance, low power, energy efficient computer to use as a server 24x7. I found fit-pc2 but then found PlugComputer such as the original SheevaPlug, TonidoPlug, PogoPlug, Ionics EMS, Marvell and GuruPlug.

I find GuruPlug Server Plus specially interesting, it's small, consumes less than 5W and costs less than $150, but I've read that it's got issues with overheating.

Do you know a good alternative? A PlugComputer or another low power, cheap computer to use as a 24x7 server?