Can i mix & match memory sizes? right now i have 2x256 sticks with 2 more slots open. Someone told me that I cant mix sizes so i was just going to take those sticks out and go with 2x512, but when i was telling another friend about the upgrade i'm doing he said to just put the 2x512 sticks in the other 2 slots I have but i didnt think i could do 2x256 and 2x512. Is he right? can I? or was the other person right about not mixing sizes?