i figured out my problem!
i'm posting it here so people with the same problems i had can fix it too!
ok the problem was i was using reiserFS as filesystem on /
that partition was around 100 gb's, i split it up in two partitions (each 50gbs) and then i made both partitions use ext3 as filesystem.
i had to reinstall all my programs and stage etc, but if you've done it before it shouldn't be much of a problem.
then just use this
to set up quota's!