Please Help! Iv just got an extra 4GB of ram for a MacPro 8-core (2x4GB chips on top of the standard 4x1GB chips) I can get 6GB to work but 2 wont show up. All of the chips work when i change them about but not the last 2GB, the slots show as "empty". I looked up to see if you can mix sizes, seems that you can as long as you keep them in pairs on the risers. Any ideas? Please..... thanks !