By looking at the manual that came with E521, you can certainly install up to 4Gb of RAM in total. Certainly, you can install 1Gb stick on each one of the four slots.
There was a live email to me that a stick of 2GB Rams worked. The Ram used was Gskill 2GB X2, PC6400, 1.8-1.9volt
Tom Ackroyd (the person whom I have spoken to) ordered the Brisbane edition of AMD 6000+ and two sticks of 2GB ram and asked me if they work all right.
The items came to him and he confirmed that the RAMs did work!! (but had a bit of trouble installing the new CPU).
He had managed to install two sticks 4Gb in total but I can not guarantee that you can install FOUR sticks of 2GB ram. I myself haven't tried this yet.
I have seen someone in some forums somewhere who installed 2GB ram but I won't trust them.
I would rather like to continue investigating this matter with Tom as I can contact him straight away.