Thanks for the link, I just did the scan on Crucial, here's a screendump.
I have added a third 1 GB RAM to the last slot. There are only three slots in my PC. Crucial.com scan tells me there are four, that I can upgrade, and that currently I only have 2GB inserted.
I have three GB inserted and I have tried to fit them in different slots, swapped them around. DXDIAG and Crucial both still say there is only 2GB in my machine!