The Intel® Pentium® D 925 would be about double the power that an Intel Core® 2 Duo would have used. But understand this is only part of the total power that your computer takes.
The biggest issue was that the Intel Pentium D's ran really hot and didnt give anywhere close to the performance that you would have gotten from an Intel Core 2 Duo e6400 at 2.13GHz.
As far as the power bill difference under the most extreme I would guess it may cost you an extra dollar or two a month. So not really making a big difference on the bill.