For those of you who might like to know:

I bought
- an Compact Flash Card Expansion Pack Plus with a 920 mAh removable battery
for about 65$ / 55?,
- a Haicom HI-305N Multi-Mode GPS Receiver with Built-in SD Memory Card
(105$ + 25$ S/H at http://store.yahoo.com/semsons-inc/hamugpsrewib.html).
One of the nice things about it is a very low power consumption of only 35
mA!

The GPS is still in the mail, so i haven't been able to work with it yet.

I will also need a mount and probably an external power source - or maybe a
handful of spare batteries for the expansion pack;-)
My iPAQ had a new battery a couple months ago, together with the extra 920
mAh battery it maybe will be enough for short flights.

Thank you all for your advises, you have been very helpful!

Niels