My desktop is an old home-built machine circa 200[5-6] running Ubuntu 11.10 (but this is not relevant because I'm reading available ram from BIOS loading screen), with an ASUS P5GPL motherboard, not X or X-SE - it has four slots. I'm mainly a laptop person, but keep this around for running a server from if needed, backing up to, seeding Ubuntu to people from, etc…
It has four (DDR) ram slots, two black and two blue, in the order black-blue-black-blue (I will call them D, C, B, and A, respectively) with some space in the middle. The blue ones are the closest to the processor. I used to have two 512MB chips in the two blue slots.
I just got a 1GB chip and plugged it into one of the black slots; my system didn't recognize it. I messed around and discovered that it will not recognize chips in many positions, and I couldn't get it to recognize all three of these chips at the same time. In particular, if I put the 512MB chips in A and B it would only use 1, but AC, AD, BD, and CD worked. I didn't try BC, I believe. Only some of these continue to work when I switch the 1GB chip into one of these positions.
Can I have some advice as to how to position these chips to get all 2GB used? How about if I get another 1GB chip - where should I put the two? And what about the RAM maximum Crucial says? Can I go above 2GB, if I get another 1GB chip?
Right now, I have a 512MB chip in A and the 1GB chip in C.
EDIT: I read some other posts and tried dmidecode in Ubuntu to clarify the max memory question, that wasn't a major part anyways. It says my max memory module size is 1024M (OK) and my max memory size is 4096M (doesn't agree with Crucial OR the Asus web site, maybe it will only work while in Linux and BIOS won't OK it?).
EDIT2: My 2 512MB chips in either A and B or C and D cause a message in BIOS that that configuration is not supported and it only uses 1. Likewise, if the 1GB chip is in another slot at this time, it will only go to 1.5GB.