Because it was so insanely cheap, I bought 4gb of RAM (Kingston DChannel 4GB Kit, 2x2GB PC2-6400, CL5 @ €60,-) to complement my current 2gb.
First attempt to install failed (I was too lazy to put my machine on its back and tried to press in the DIMMs sideways, so they probably wheren't seated properly).
The second attempt, only 3gb of RAM shows up in the BIOS. Closer inspection reveals in some sub-menu of the BIOS (approximately):
Mem. Total: 6144 Mb
Appropriated: 600 Mb
Available: 3100 Mb
Googled this and found on this forum someone with the same motherboard (Asus P5W DH Deluxe). I was already thinking of upgrading the BIOS, when I was mighty disappointed to read that 32-bit OSs only support up to 4gb of RAM.
Checking the free memory in Ubuntu (Hardy Heron) resulted in 3100Mb (using sudo free -m ), but using dmidecode -t memory shows all DIMMs properly installed in their sockets, with the right sizes.
However, the post also mentioned a BIOS setting enabling "memory remapping" for 64-bit OSs. I gave that a try, and indeed, the BIOS counts up to 6100Mb of RAM, during POST.
Unfortunately, Ubuntu did not agree. It still shows 3gb. A detailed explanation on why only (up to) 4gb shows up in the OS is explained on codinghorror.com.
Looking even further, I stumbled upon this forum, where scanning the posts I read this: "I've never heard of the hardware issues with PAE, but I know that's the way to get the system to see all 6GB." Could it really be? Simply enabling some feature could make my fresh new RAM work?
Of course not. PAE could solve it alright, but it won't be easy. A bit discouraged by the codinghorror.com story, I feel a bit relieved reading that Linux kernels will support up to 64gb of memory on a 32-bit machine on wikipedia if one compiles their kernel the right way, but I still don't know how. Enter The Almighty Ubuntu Community Site. There, somewhere in the paragraphs, it is even mentioned explicitly that "Ubuntu modules source may also be needed if you plan to enable PAE and 64GB support in the kernel for 32bit Hardy (8.04)".
Since I'm not quite an expert on recompiling kernels (I did it a couple of times when I still ran Slackware on my old server, but I didn't know what I was doing back then), I turned to How To Compile A Kernel - The Ubuntu Way on howtoforge.com. This highly complete description makes me feel confident even I might attempt to recompile. Someday, that is, when I can't screw up my PC because I'm tired and pissed off because I just wanted to upgrade my RAM, something I did many times in the past 18 years without any problems...
To be complete: the alternative to recompiling PAE into your 32-bit kernel, you might want to install a 64-bit OS. I don't know. Maybe I'll have to read the Advantages and Disadvantages of 64bit topic some day. by Martin @ 0:07 4 October 2008