Question high memory usage

To add to @rgd1101's post - get a compatible 2x4GB (8GB) kit.
Even though OEM motherboards are VERY forgiving when it comes to mixing RAM, your chances of it working perfectly, at rated speed, aren't very good if you just get any RAM. If you try to match your current 4GB stick your chances improve somewhat but are still not the best. If your current 4GB is made up of 2x2GB sticks just toss them and get a compatible 2x4GB kit.