Have an x58 Classified e760 board that runs 12GB of Corsair 2x6 XSM memory just fine, manual settings and XMP.  I also have a set of GSkill Ripjaws 12GB 3x4 that the board wont use more than 7.99GB on XMP or manual settings.  It sees 12GB but says only 7.99 usable.  Any idea what im doing wrong here?

I wanted to mention , I also followed the guides, up to RMA for the motherboard. This was initially done using an i7-950, I later upgraded to the i7-970.

Ive tried everything i found when i was searching for a solution including testing each stick, running memtest (i let it run over a weekend I was out of town), Ive tried stock manufactures settings, default bios settings, XPM, and Ive also tried several manual settings.  Ive inspected the cpu pins, Ive even tried it in an e760 board and a classified 3 e770 board and same thing happens.  one i7 950 and the e770 board has an i7 965.  The corsair set of 6 sticks works fine in both boards, the gskill set of 3 sticks isnt fully used on either board.  The cpu pins are fine on both boards, I took a magnifying glass to both.  Running latest bios 83, ive even contacted gskill and had them walk me through their bios settings.  Its odd that my system sees 12gb but tells me specifically (7.99 usable).
 
Ive re seated everything on the board. The boards visual appearance looks fine as far as i can tell.

I diddnt see where it stated the amount of ram in the bios.  I reinstalled the os in both machines. Win 7 shows 7.99 usable every time.  Ive put the slower corsair ram back in and I havent had any problems.  

UPDATE: After changing the CMOS Battery (which hadn't been changed since I purchased the board back in Oct. 2009); after unplugging the AC. After putting the new battery in I pressed the CMOS reset button and plugged the AC back in (after putting in my first RAM stick to test), taking the RAM sticks out again and testing each of them in the red slots. 
 
I now have 12 GB in the BIOS and 12 GB usable in Windows.
 
...maybe this will help you :D
 
 
Original Post for reference:

Oh-my-god!
 
I'm not alone.
 
OS: Windows 8.1 Pro
RAM: 12 GB Corsair Dominator GT
MOBO: X58 Classified (760 I believe)
BIOS: E760_83
 
My system also has 12GB, yet only sees 8GB (BIOS). One would say "oh the slots/sticks are bad". However. Windows sees how much is installed, but says only 7.99GB are usable.
 
DXDIAG says 12GB; E-LEET says 12GB; Speccy says 12GB; AIDA64 says 12GB...the latter two tell me exactly the RAM's settings (x-x-x-x) and what brand they are.
 
It appears DIMM 3,4 do not report memory. As I've tried a 2GB and 4GB in those slots and the sticks are seen in the BIOS, but are seen everywhere else....just not usable :(
 
Tried everything I could think of short of reinstalling...which I don't see the point in doing that as the BIOS doesn't see the RAM.
 
I'm really thinking the slots died because I use to have 12GB usable...and one day I just noticed that Adobe only reported 8GB in it's 'Memory Allocation' section for Premiere.
 
Haven't touched any of the CPU or RAM in...jesus...4 years I think. So 'bent pins' is impossible unless gremlins are at work....

If he had 32bit it would only show 3.99GB because 32bit can only address up to 4GB.

It's likely the same issue I had. Which appears to have been resolved by simply changing the CMOS battery and pressing the CMOS reset button (for the third time during this crisis) O.o