Couldn't find one asking this specifically... Currently 4X2gb modules. Thinking about adding 2X4gb. This would leave me 2 slots so I could add another 8 gb later and have a total of 24. Seems like 4x2gb and 4x4gb should be fine? Until then I would either have 16 or 18, depending if I filled the 2 slots with my 1x2gb stock ram. Guess I want to know about performance loss with not using powers of 2, and also not an even amount of matching pairs. Thanks.