There's your problem! You are using a 32 bit OS, which only supports up to 4GB of RAM (a little less since the OS is using some) due to the 32 lanes. Even if you add more RAM, the OS can only use 4GB. You will have to upgrade to a 64bit OS to use the other 2GB of RAM. As for max RAM for 64bit, it is in theory 2^64 bytes, but no home computer can have that much, so the limitations usually fall on the motherboard max, which in your case is 32GB.