i just got a 15" i7 w/ 4gb of ram, and since i might want to upgrade to 8, i went to the apple website and i see that they only offer 8gb available for sale? does this mean I will have to get 4GB somewhere else, or the new MBPs only offer 2 configurations (2x2GB or 2x4GB) and not 2 (2x2GB)s . I might sound confusing 