Finally I've made it. Got me a G4 Cube. There is one little problem: this tiny bastard won't start up!
I've tried everything, from resetting the PMU over zapping PRAM and reseting a couple times to swapping some parts. Nothing seems to work, all I get is a spinning HD and CD-Drive and a wonderful LED lightning up, but that's less as I was expecting from it to do... 
Has somebody any experience on that kind of problems??? Chris501
just realized that there is a small green LED on the mobo, called DS1, it goes on after pressing the power button and goes off after about half a minute or so...
You can download the Cube's Service Manual here.
http://www.whoopis.com/computer_repair/
Could be a dead internal memory battery. Check this site for battery part numbers and sources.
Mac PRAM, NVRAM, CUDA/PMU & Battery Tutorial
http://www.geocities.com/texas_macman/pram.html
Cheers, Tom
Mac Troubleshooting, Maintenance & Tips
http://www.geocities.com/texas_macman/MacTroubleshoot.html
Thanks for your assistance, Tom! I'll check it out on Monday when I'm back at work.
One thing is for sure: it's not the memory battery, already thought of that but it seems to be ok (3,6 Volts).
Thanks again, Chris
Thanks again for the links, but it seems that the mobo is really dead. Everything else works fine, beginning from RAM over battery and HD, even the graphics card is working perfectly... seems that this will get very expensive!
Chris
Have you tried resetting the CUDA switch? That may help.
CM
www.cubeowner.com
They may have the answers. And you may have damaged (or have someone else damage) the VRM board. Or it may have been replaced. Or whatever. You could try getting another, to see what happens?
thanks guys for all your help! but the main logic board is completely dead, and a new one costs over 450 euros (i guess at least 480 american dollars), and that's way to expensive. i'll go and get a mini for that!