UPDATE OF THE PREVIOUS POST:

Finally I found a way around without risking my n810 at all, for anyone interested:

what you need:
- a computer with linux
- resize2fs on it
-> to install it ( normally in root or with sudo):
--> debian and ubuntu: apt-get install <program>
--> gentoo: emerge <program>, I think, it has been a while since I last touched a gentoo box)
- optional: SD card reader + adapter to (micro/mini)sd depend of which of the 3 you use.

steps:
1. Either connect your IT to your pc or get the card out (caution: do not take it out if in use, safest way to make sure: power off your IT) and put it in the card reader.
2. go in the terminal or if in X launch a terminal emulator
3. type cd /<your path to your (micro/mini)sd/
4. e2fsck -f <your debian image file name>
-> it check if everything is allright with the filesystem
5. resize2fs <your debian image file name> 1950M
(As psykosis said you can change the size but if your (micro/mini)sd card is formatted in fat, like mine, the file better not be bigger than 2gb)

And that's it, a loooot more room for installing cool apps

thank you very much for the resize2fs trick psykosis I wouldn't have found it myself.