I have a Precision T3500, Xeon X5675, 24GB RAM (6x 4GB PC10600 non-ECC), BIOS version A17, Windows 7 Enterprise SP1 installed. I've been trying to get the system to make 24GB available, but almost every time I boot I find that only 16GB is available.
I've read quite a bit about this online and found that sometimes the BIOS is an issue. The system had A13 when I first started working on this and upgraded it to A17. It actually complained that I couldn't go directly from A13 to A17, so I did A14 -> A16 -> A17 and after this when it booted I saw that 23.5GB ram usable (hurray!), but the next time I rebooted it went back down to 16GB usable (doh!).
So it must be possible, but why didn't it stick?
Since then I've tried changing a variety of settings including the myconfig boot menu max memory setting (which when I reboot no matter what number is there it shows up as 0).
If I don't get this working I'll probably just drop it back down to 12GB and use the other 12GB in another system, but it doesn't really make sense that it can't always boot with all RAM available, especially since I saw it happen at least once.
I did some more experimenting and found that if I reset defaults or change any settings in the BIOS and hit Apply then the first time I boot Windows will show 23.5GB usable. If I reboot again usually it will go back to saying 16GB usable. So Windows must be changing something behind the scenes.
Also it seems that the system is less stable when 23.5GB is usable. I've had it randomly reboot suddenly (not even a bluescreen) in this configuration. The Windows system log says that the system restarted unexpectedly and there is another message that a crashdump failed after some system reboot messages. It hasn't yet happened when it says 16GB usable, though I suspect the video card (Geforce 8300GS) might be a bit flaky and will be swapping that out.