I just got a new mac pro with 12 Gb of ram. This means the 4th memory slot is empty and wanting to be filled asap. Now what fits in there best? Of course it has to be the same type that's in the other slots, so i'll be going with the Apple stuff. But does it have to be a 4 Gb module, or can i put in an 8 Gb or even a 16? Are there any downsides to not having "paired" slots?