Same issue here. Michelle is right, on linux perfect, with unfinished botch-up (lol Michelle) aka window$ lots of troubles. My comp is not underscored, its a 2x hexacore xeon X5660, 32GB of reg. ddr 3 1600 and a nVidia GTX 680 with 4GB memory, main os fc18_x64. I agree that window$ can not handle memory sizes over 3GB even you have more installed I maded THE TEST and window$ hung up and even slowed down with over 48GB, linux is able to allocate the maximum of 192GB ram without any speed decreasements my mainboard can handle.
ciao window$
@Kwaki, don't mixup facts who have been already explained.