I've just purchased a 1GB stick of memory with the intention of upgrading my PBG4 (rev e) and just as I was reviewing the order I noticed that the stick I bought is faster than the ones currently installed, so I did a search here to check if it would work or not, and read that RAM has to be installed in identical pairs...? Now I'm quite confused and wondering if someone could clarify (did a Google search also but just got bombarded with cheap memory offers). Here's my current set up: 2 x 512MB SODIMM PC2-4200S-444 (not sure what the 444 is - I think they're 533mhz) And my proposed set-up: 1 x 512MB SODIMM PC2-4200S-444 1 x 1GB SODIMM PC2-5300-667Mhz So, is this going to work, or do I need to buy another 1GB stick to match? Update: Have just read a couple of things that suggest that matching sizes is not essential in the PBG4, and the performance loss is very small compared to matching. Still can't find anything about the different speeds though.