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?

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?

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.

There are 2 probable reasons for this.

• You need to turn on Memory Remapping in your BIOS
• Your video card may be sharing internal memory.

It is usually the latter that is the case. This was one of the planned features of Windows 7, you can read up on it here.