With my new P5B and 4 1GB DDR2 Geil PC6400 my motherboard just recognize 3GB RAM.
0211 is my BIOS version.
I tried 3 1GB and 1 512 (3,5GB) but not, just 3GB in BIOS.
There is apparently trouble with 64 bit OS's too. In searching for a solution to a post on the same issue in the memory section, I found a lot of people with this issue but no solution.
i saw something in the latest bios for p5b "Support x4DIMM" but preapare urself if on planning to upgrade bios, read about the versions from other people