Ram not being recognised

Received refurbed XPS 8100.  Started out with 6gb RAM (2.96 usable).  I read a few threads, said try reseating ram.  I reseating all, and switched the 2 gb sticks with each other.  Now I get 6 gb RAM (5.96 usable), 3.75 available.  The only way I see this happening is I have one bad stick, and one bad slot.... or one of the 2 gb sticks wasn't seated to start with (although all were and are "clicked").  I've read several threads about MB's being replaced to solve the problem. 

Should I check anything else before spending hours with support?  BIOS version A03, GTS 240 1 gb video.

Edit:  I solved my problem.  After I did the above, I followed this to item #4:  http://www.sevenforums.com/tutorials/66482-memory-set-maximum-amount-used-windows-7-a.html  I now show 6 gb.