The question how many energy modern and a little bit "older" systems based on the AMD Barton or Intel Northwood core consume is an interesting question and SilentPCReview has dedicated a whole article to it. They have tested systems with the following processors: AMD Socket 754 Athlon 64 3200+ (Newcastle), Intel Socket 478 P4 2.8 GHz (Northwood), AMD Socket A Athlon 2500+ (Barton), Intel LGA775 Pentium D 820 (2 x 2.8 GHz), Intel LGA775 Pentium 670 (3.8 GHz), AMD Socket 939 Athlon 64 3500+ (Venice)
The following questions are addressed:
How much AC power does a computer and all its related peripherals (monitors, speakers, and printers, for example) draw?
How much AC power does a computer draw apart from its peripherals?
How much DC power does each individual component in the computer system draw?
How much power does a computer system draw from each DC voltage rail on the power supply?