It's kind of a trick question because there a million variables involved.

Which CPU?
Which HD? WD Green Hard Drives use less...
Which cooling? Intel stock fan or ICE module?
Anything in the PCI slot?
What USB devices will be connected?
etc. etc. etc...

30 Watts is at idle. So the PC is on, but you're not doing anything.

55 Watts would be every USB port in use, a power sucking Pentium Dual Core, the Intel stock fan on high, and the hard drive chattering away.

For keeping it low...the Core 2 Duos use less than the Celeron Dual Cores but they use less than a Pentium Dual Core. The ICE module uses considerably less power than the Intel stock fan. A Western Digital power saving hard drive can use about 33% less power than a typical drive. Two hard drives would use double the power; a DVD drive would draw power, etc.

That's why the range of 30-55W is given. A typical, in use, stock K45 runs 40-45 Watts, though.

Hope it helps!
Dan