3GB of RAM showing up as 2GB

If the BIOS is reporting 2Gb, then it's a hardware or BIOS problem, nothing to do with XP.

First thing I'd do is try the RAM two sticks at a time to see whether they're all good. Second thing would be checking if a BIOS upgrade is available.

Double check what the BIOS is reporting. I suspect that your memory is actually there, but remapped over the 4GB limit: a 32bit OS will be able to see only 2GB, the BIOS should report the correct amount of memory though.

Your memory map might something like:

0-2GB physical memory
2-4GB hole for MMIO
4-5GB physical memory

My P5B-E does the exact same thing, but I'm running a 64bit OS...