I bought a Supermicro 5018D-FN8T: The Chronicles [Edited Title]

[chrcoluk]

if you want to have least impact on performance with best power efficiency I wouldnt let the cpu go to idle clocks, but keep c-states enabled, c-states save's far more power than eist and c1,c2 are both very quick and cheap for performance. c3 is a jump up from c2, but a trick is to only enable c3 on say half the cores, so half the cores will still respond instantly for interactive stuff, whilst c3 will still wake up quick enough to deal with loads that need all cores.