>With a linear regulator you'll have about 100
    >Watts of heat to dissipate to get 25 Watts of
    >useful power.

    I'm curious: How did you come up with this? Is there a rule of thumb
    to use for linear regulator efficiency?

P = IV, although it's not quite as intuitive with an active device
like a voltage regulator as it is with a simple resistor.

If your input is 25V and your output is 5V, you have a 20V voltage
drop across the regulator.  If you're delivering 5 amps, that's going
through the regulator as well.  So you have a "black box" device with
20V across it and 5A through it.  P = IV = 5*20 = 100 Watts.
(depressing, eh?)

BillW