I got curious about this issue and did some Google searching.  It appears 
this is not an unusual situation, though I've not heard of it previously. 
This anandtech forum thread offers some insight into the problem.

http://forums.anandtech.com/showthread.php?t=326801

From reading that thread, I gathered that there should be a BIOS setting for 
something along the lines of enabling "memory remapping".  This will need to 
be enabled before installing Win7 64-bit.  I looked through the online 
manual for your motherboard and didn't see anything pertaining to this in 
the BIOS section.  Hopefully, your current or newer BIOS will have something 
that is the same or similar to "memory remapping", even though the manual 
doesn't cover it.  (I see they've made 5 BIOS releases in 2 months, 
suggesting that the consumer is their "beta tester" for this product. 
;-)  )

HTH

John Sproule

---------Original (Abridged) Message Below-------

Date:    Wed, 4 Nov 2009 18:43:42 -0600
From:    Dean Kukral <[log in to unmask]>
Subject: Memory not available?

I am in the process of building a new computer based on an Intel i7 870, an 
MSI  P55-GD80  1156 motherboard, and four two-gig sticks
of Corsair memory.

When I booted the computer into the BIOS, it says on the first page at the 
top that I have 4GB of memory, even though I should have
8GB.  In the bios, it "sees" all four sticks of memory.  It seems to read 
the memory properly per the SPD.

Then, I loaded  Windows 7 64-bit software and looked into the System 
settings and it said that I had 8GB of memory, but it also said
that I had "4GB reserved" by the motherboard (or bios or something).  It was 
only using 4GB.

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