I have an ASUS A8N-SLI motherboard with an Athlon 64 X2 processor.
"uname -a" outputs the following:
Linux hostname 2.6.31-16-generic #53-Ubuntu SMP Tue Dec 8 04:02:15 UTC 2009 x86_64 GNU/Linux
I have 4GB RAM installed in 4 1GB modules. The motherboard BIOS is the latest version as far as I can tell (but still four years old.) The motherboard only sees 3.2GB at POST. The BIOS setup has a pair of options which change this- essentially a "hardware" and a "software" version of PAE. (I don't recall the exact wording, but I will come back and post it after I reboot the machine later today.) If either of these is enabled, then the board sees all 4GB at POST and all 4GB is available for Ubuntu to use. Unfortunately, enabling either or both of the aforementioned options causes both onboard LAN adapters to stop working.

The board also exhibits other flaky behavior which I won't go into yet but which I suspect may be BIOS-related. I've been considering using coreboot / LinuxBIOS on this machine to work around these issues, and I wonder if anyone here has experience or advice on the subject.