We're using a particular Tyan board on many of the workstations at work. It maxes out at 4 RAM sticks of 1 GB each, for a total of 4 GB. However, upon booting, the OS (and memtest86) would only find 3296 MB (despite the BIOS claiming "4096 MB OK"). I updated the BIOS and that let us have 128 MB more, for a total of 3424 MB. A friendly Google search yielded information that suggested we enable the "software memory hole" in the BIOS setup; apparently, these things reserve space between 3.5 GB and 4 GB for god knows what, and enabling the setting frees that up. Upon enabling the SMH, the RAM test during bootup shows it happily counting up to 3072 MB, freezing for a second, then counting up from 4096 MB to 5120 MB, and upon reaching that, going back to display "4096 MB OK". The OS now admits to seeing all the RAM.
(Oh, and i didn't even go into the near-impossibility of making a bootable CD that included the BIOS Flash file and utility that worked. We ended up finding a floppy drive and disk rather than suffer and keep dumping coasters.)
It's a good thing that my livelihood doesn't depend on understanding computers, because god damn i hate the fuckin' things.