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I might have discovered the problem. Snow Leopard and previous versions boot into 32-bit mode by default. The main reason for this is compatibility with 3rd party driver software. There are many more details about it googling for "snow leopard 64-bit".
SixtyFourSwitcher is a free PrefPane that let's you switch between 32-bit and 64-bit easily. (I have booted the system with 64-bit kernel extensions enabled and so far have not discovered any problems.)
In 64-bit startup mode, VirtualBox has no more "HostMemoryLow" issues and finally it allows me to set the memory of the Virtual Machines up to 16 GB, which is very useful.