I have Exactly the same problem!!!
when i tried to update the bios it gave me this message
and then i accepted it...
after that when i restart the system, in the first screen of booting the system it says that i have 2048 mb!!!
and i tried to even downgrade my bios version but it didnt helped...
its ridiclous...
in win 7 x64 i have 4 gb ram and 2gb usable...
the way i figuered it doesnt work on Dualchannel mode but if install the memories in for example 1&3 slots or 2&4 slots its working fine! but why?