In the BIOS I saw,
Memory Hole at 15M-16M [disabled]
I enabled it and CentOS would not boot up, "Error 28: Selected item cannot fit into memory". I was able to boot into XP Pro, but I stll saw 2.67 GB showing as useful in My Computer.
It the BIOS I also see,
DRAM Timing Selectable [BySPD <or> manual]
There are a series of options available that are greyed out unless "manual" is selected.