Check that you have a 64 bit OS like Rolli said.
If you do, then go to start, and in the box type MSCONFIG, then go to the boot tab and hit advanced options. If there is a box checked there that is limiting the RAM to 4 GBs, get rid of it.
If neither one of those things is the problem, then try to start the computer with one stick at a time. I recently saw a similar problem where the problem was a bad RAM stick.
In regards to the last part there, go into the BIOS and see if it is listed as having dual channel active. If you have the sticks in the right slots and you can't get dual channel mode active, it is probably bad RAM.
A program like MemTest86+ can also be run on each stick individually with the other removed in order to make sure they are good sticks.