Honestly I believe a friend of mine had this same exact problem..However I really would feel bad for you to waste time/effort if this does not work. For future always clean install.
If you have some time maybe others can come in to give advice on what to do. It is odd..that two sticks of 4gb=8gb become 6gb
Anyways..It could be a variety of things. Bios settings,ram needs to be put in properly (sometimes they are sensitive),buggered slot, a bad stick. As said above test each stick to make sure windows actually reads the ram properly first.Make sure memory is not defective. Also make sure everything is updated..Bios,drivers etc. Sometimes even reseating the Cpu. Can also try deselecting maximum memory in msconfig which I often hear works.
If all else fails.. Then only option is clean install which is no guarantee. Note that usable is always lower then actual physical ram.