I have an HP Kayak XU 6/450 that started life with a single Pentium II 450, 256MB RAM and a 4GB SCSI HD. It has since been upgraded to a dual Pentium III 550Mhz with 384MB RAM and many HD's.
I recently came into some 512MB sticks of PC133. The stated limit for this rig is 512MB (4x128MB). I decided, just for a larf, I'd see if she'd take 'em. I installed 4 sticks of 512MB and she wouldn't do anything. No video, some disk activity. When I replaced the first 512MB stick with a 128MB stick, I'd get video and Windows would try to boot, but it would give me the "Can't load pci.sys" error, with the option to repair it from the XP disk. This error completely disappears if I remove the 512MB sticks.
At this point, most people would just say "Oh well, it doesn't support that much memory." and move on. But, something is puzzling me. When I start up the computer and it initially does the RAM count, it gets to 128MB, effectively ignoring the 3x512MB sticks in slots 2-4. However, when I get to the "summary" screen, it displays the memory as being 1664MB, with 128MB in the first slot and 512 in each of the other slots. I don't understand why, if the computer can correctly identify the memory modules, that it can't use them.