Why can't Windows 7 x64 use all installed memory?

Why can't Windows 7 x64 use all installed memory? I'm running Windows 7 x64 RTM on a Dell Latitude D520 w/ 4gb memory. The system control panel reports:
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Total answers/comments: 2

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Guest [Entry]

Either you do what Post#1 suggests, or your motherboard simply doesn't support the memory address mapping, which means: No 4GB for you with this hardware.

Guest [Entry]

It may be a windows limitation, too. However there aren't usually on pre-release version IIRC.