I heard that an Athlon XP mobile need much less power than a "normal" Athlon XP.
That would mean the cooling fan could be smaller resp. could run at a more silent speed.
Is this true?
Yes.
How much power can be saved when using a mobile verion (on a desktop motherboard)
in comparison to an equivalent desktop version?
That's impossible to say. It might be 5% less power, it might be 75%
less power. Both desktop and laptop chips come at a variety of rated
power consumptions. Different speed chips at the same rated power
consumption will also have different real-world power consumption. In
fact, individual chips of the same rated power consumption, same speed
and even the same wafer will have different real-world power use. Also
there is a big difference between power use when the chip is idle vs.
when it's working at full load.
Then there is also the question of whether or not the desktop board
supports some of the more advanced power saving features of the mobile
chips. AthlonXP Mobile chips support a feature called PowerNow! This
lets them dynamically change the processor speed and voltage according
to system load. Very smart idea given that most computers are idle
about 99% of the time, even when they are being actively used (ie if
you're typing an e-mail the computer is spending most of it's time
sitting around and waiting for the next keystroke). Unfortunately
this feature was not supported on the desktop AthlonXP and therefore
not supported by most desktop motherboards. Without motherboard
support the power savings from the mobile chip are usually much less
than they ideally would be.