An update: I tried CentOS 6.1 (the only one i had at hand) and it gave the exact same result as Debian.
I have in the meantime tried to use other RAM modules, and unfortunately they also did not give more than 3.5gb. Considering that i pulled the ram from a 24/7 stable system i assume that it is indeed the motherboard/BIOS that does not function properly. I will return the motherboard and see if i can get one that works. Thank you all for your ideas and comments, I will post an update later on to tell how the new motherboard functions. best regards, Mohtadin
