I have been on the same binge.  Power is getting to expensive and I have a bad habit of building all these servers that do varying levels of work out of old boxes.  I spent a few random $$ on the F/S forum and updated my E6600 to a Q6600, added a bit of ram to it and managed to use hyper-V to p2v all the old crap to one box.  So for about $80 total in used parts I shutdown a couple of computers, 1 being a dual dual p4 era Xeon 3.2ghz with that had FB-DIMMs.  I am waiting to see what my bill says the consumption for the month was.
As a neat bit of FYI... The Xeons did have speedstep but it was nonfunctional. These where the P4 era Dempsey 130w TDP chips.  The machine had a Dell 1000watt psu.  Measured at the wall that removing 4 of the FB-DIMMs dropped the power consumed by 45watts. Thinking about it... I could have likely spent the $650 for the ivy bridge kit I am looking at buy to replace the q6600 and come out ahead in the last year.