Usable Memory

I have a problem with my Alienware Aurora R1 desktop.  In the Windows 7 control panel it shows that there is 9GB RAM (5.99GB Usable).  In trying to solve this problem I have updated the BIOS, and reseated all of my RAM.  I have Windows 7 x64 Home Premium, so it has a memory cap of 16GB.  My RAM configuration is set up as 2 sets of 3 RAM modules with black/white tabs. [ (b)(w)  (b)(w)  (b)(w) ]   While rearranging/reseating the RAM I have found that the third memory recepticle on the motherboard does not read the memory attached to it no matter the configuration of the modules.  Since I have given up on trying to fix the problem and gain access to this last recepticle, would there be any problem in only using the other 4 memory recepticles to run my computer?  My computer is set up to have RAM in sets of three, but it has been working fine with 2 sets of 2 only.  Also, I can reach my RAM cap for Windows Home Premium using only these four recepticles. (4x4GB)  If anyone has a solution for either the "Usable Memory" problem or an answer to my question about only using the 2 pairs of memory please I would truly appreciate it.  I have been working on this problem for quite some time now.

Thanks,

Chris