I have a desktop homebrew, I've had for several years with a MSI K8NGM2 MB.
XP has been bogging down recently, running low on RAM. Right now simm1 has dualddr 512mb. I purchased a known good second, identical memory but no matter where I put it, it doesn't show up.
Either stick, by itself, works fine but when the second is added it doesn't show up in bios check or XP.