Don't know total answer - but the power supply article here on SPCR has some data that might help a little.
Unfortunately no Core2Duo rigs there, but the P4 rigs listed run from
(Edit - fixed numbers, originally posted DC draw, not AC).
I know it is compare apples to oranges
- but one measurement found
MacMini Core2Duo runs about 35w at load (Folding).
Specs put the older MacMini CoreDuo idling about 20w.
A typical Core2Duo probably lot more power than that.
One Core2Quad (2.4GHz) system is mentioned idling at 66w.
(Data from the PPDPW discussion on the SPCR folding forum.)
[Of course this is only part of the equation - have to also consider power/resources involved in making the thing, and be sure to
turn the whole thing off when not using it. (Use WOL, timed wakeup, X-10, ...)]