I am trying to add a little more ram to my PC. Right now it's got 4gb, 2 1gb cards and 1 2gb card of 6400U. I read somewhere that 6400U and 4200U are compatible, so i put a 4200U card from an old PC in the empty card slot. However, when I powered on and checked my ram, I was still only showing 4gb total. What gives? Is there a reason my machine isn't recognizing the the 5th gb? are 6400 and 4200 actually not compatible?