I'm not sure 8GB DDR3 RAM sticks are available. From my searching all I can find are 8GB kits containing 2 4GB sticks, giving a total memory total of 16GB using 4 sticks in the four slots. I've also heard that Samsung recently started manufacture of 8GB RAM sticks, however, they don't appear to have hit the retail markets yet.
Keep in mind that the more RAM slots you populate, and the larger the capacity of your modules, the lower the maximum speed you can achieve with a given voltage and timings. At least that's been the case in the past.