I just installed another 256MB stick of Crucial PC2100 and the system only says 256MB is installed (should be 512MB). THey're both 2 bank modules and on an MSI KT3 Ultra. I've tried many combinations of moving the modules around and cleared the CMOS. I tested the modules individually and they each work fine. Are there any particular problems with this motherboard and multiple memory modules?
I finally got it working. The two slots I tried it in were holding the module really loosely (when fully locked down and secured). I just had to wiggle it around a little then it finally worked. Then I tapped it again and it stopped working.. then I wiggled it around again and it started working. Weird.