64 bit - 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:

System type: 64-bit Operating System
Installed memory: 4.00 GB (3.24 usable)

Why isn't all the memory usable? Since I'm running 64-bit, is this a dell limitation?

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Edit:

It's given me more questions:

1. If the system reports that I can use 3.24GB, does that mean that 760MB is being used by the video card? That seems like a lot, and the specs on this page state it will use up to 128MB)


2. If 760MB isn't being used by video, is it being wasted?


3. Does running 64-bit windows help me at all? If not, are there other advantages to 64-bit windows?

Answer

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.