I think I know the answer but I wanted to check. I have an XPS 400 that currently has 2 GB RAM which is 4 512MB chips. I ordered 2 1GB chips from Dell and when I installed them replacing two of the other chips I got the beep beep and no boot up. Is it because I am unable to run 2 1GB and 2 512MB chips in the 4 slots together? Meaning to upgrade I'd have to get two more 2 GB to have 4 matching chips in the slots? My computer is upgradable to 4GB according to my specs. Thanks for the input!