Read here:
http://en.wikipedia.org/wiki/Multi-c...y_architecture
A motherboard or mainboard if you will in this era is either dual channel or quad channel, mostly.
Meaning, if it's dual channel, the best performance you'll gain is by using two dual channel memory sticks that works together.
Some motherboards has quad channel, meaning you need 4 memory sticks of the same type, meaning that all 4 sticks works together.
Quad channel memory is more or less a gimmick though as it doesn't really give you more performance over dual channel, it's just how it is.
If you buy a dual channel motherboard, buy either 2 x 4GB to get 8 GB total ram, or 2 x 8 GB to get 16 GB total ram, though 8 GB sticks cost a bit more.
If you buy a quad channel motherboard, buy 4 x 2 GB sticks to get 8 GB ram, or 4 x 4 GB sticks to get 16 GB ram.
Most ram come in dual or quad channel kits when you buy them nowadays anyway, the thing you need to check though is if your motherboard is dual or quad channel, and no, you cannot change it on a motherboard, it's either dual, or quad, for most motherboards today.
Edit: You also have triple channel memory, though I feel it's just a bit silly that one, merely my opinion.
Those will be 3 sticks of ram working together so you either have 6 GB or 12 GB ram, though I think less motherboards today has triple channel memory.
I believe all socket 2011 motherboards has quad channel only, though I might be mistaken =)