Something I read in the AMD specs, which was not that clear, indicated that the memory controller on the Athlon 64 chips only supports up to 4G. I believe Opterons and maybe FX chips support more. I think you need registered memory, not the normal unbuffed stuff, for Opterons. IAC AMD does brag up the amount of memory Opteron servers can use, and they use numbers like 256G.
Since normal Home Windows runs in plain 32 bit mode, you can't access over 4G. In fact 2G is reserved for OS use (like virtual memory) so any one program can't use over 2G. Server Windows is probably different, and uses a paging scheme built into x86-32 bit to get to over 4 G, and Windows 64 bit is definately different. In 64 bit Windows, every 32 bit program will be able to access 4G, and of course 64 bit programs can access more. But that's assuming the mobo can take over 4G. Otherwise, over 4G will be virtual memory.