Since you're buying a dual channel motherboard with 4 memory slots, then you should buy one or two dual channel memory kit(s). If you bought a motherboard for an i7 (X58 chipset and six memory slots), then you would buy one or two triple channel memory kit(s).
1. Would it be faster to do 2 x 8GB or 4 x 4 GB (assuming Dual Channel in both cases)?
2. How does Dual Channel work with 4 slots. Is it two seperate dual channels, or does each channel span 2 slots?
3. If it's two seperate dual channels, would 4x4GB dual be faster than 3x4GB tripple (only talking about speed, not capacity)
Provided the motherboard is able to run the same timings and speeds for both setups, the difference between a 2x8 or 4x4 will be minimal on artificial benchmarks and undetectable by a human being. Having said that, know that from an electrical perspective 2 Dimms are easier to drive than 4.
Look at the RAM Recommended list and the 1600 addendum. Note the 3 columns at the right which refer to whether you can put 4 or only 2 sticks in the slots. There is a limitation on AM3/AM2+ design which limits some ability to use all four slots at full top speed - I don't know all the details on this.
But to make sure this doesn't affect you, if you want 8GBs of RAM, then get a pair of 4GB modules. If you want 16GBs, get a pair of 8GB modules. Don't use 4 of anything if you can avoid it. You'll likely end up with RAM that runs faster. (When the RAM guide mentions a kit of 3 - it's just stupid. That means that they only had the 3 stick kit on hand to test - not that you can use 3 of the sticks!)
This board doesn't support both dual and triple channel RAM. On an Intel-based board with a choice, it must be hard to measure, since you would never have the same amount of total RAM. That is, you can't compare dual-channel with 4GBs to triple channel with 6GBs. The 6 will be "faster". Likewise, dual channel with 8GBs will be "faster" than triple with 6GBs. I really don't know the answer if it can be quantified.
And there is no such thing as one triple channel. Gotta have 3 sticks to make a triple. If you put 3 sticks in a dual channel, then you lose dual channel ability. Notice the RAM guide for board shows only 1 stick, 2 sticks or 4 sticks - never 3.