Triple-channel will give you more memory bandwidth. But dual-channel DDR3 already has very high bandwidth; most real-world applications won't saturate it.
A more tangible concern is having enough RAM to run your apps, and having plenty left over for disk caching and pre-fetching, which also improves performance. So that's easy: the more RAM the better, up to the point where you have "way too much", and you have a bunch that isn't used for anything. (Above 4GB for non-server Windows, you need to run 64-bit.)
For example, suppose you have a game that only uses 1GB of memory while running; and the entire disk footprint (the levels etc) of the game is 1GB. Let's say that the baseline for the OS, doing nothing, is also 1GB. That means in total you only need 3GB of RAM. More won't help.
If you're going back and forth between levels, eventually they'll all get read in, and stay cached, as long as nothing else needs RAM, which would force the OS to clear stuff out of the cache. In this situation, you might be able to tell that triple-channel is faster, if the levels load by copying a huge amount of data from disk -- which is now cached -- into the game's working memory. (As opposed to a game that incrementally loads levels as you move around.)
But consider this: dual-channel DDR3-1600 will do 25GB/s; triple-channel will do 38GB/s. Can you tell the difference between 0.04 and 0.03 seconds? If the game used ten times the memory, you might be able to tell. What seems to have happened is that memory speeds have outrun memory sizes. Aside from truly "memory-intensive" applications, you don't use enough RAM to appreciate how fast it is.
So to go back to your question: if 6GB is "way more" memory than you need to run your game (and nothing else), then in theory you will get the best possible performance with that triple-channel configuration. But it may be best only by some imperceptible margin, with dual-channel being almost as fast.
If the game uses anywhere near 6GB, or perhaps more importantly, if you use anywhere near that much just "doing stuff" on your computer besides or simultaneously with gaming, then get 8GB.
I'm not aware of any performance differences between configurations #1 and #3. The factors there are price and future expandability. 8GB seems like a "good" amount of RAM for the near future. If price is not a primary concern and you want some future-proofing, then you could get a triple-channel board and two sticks. Then adding a third stick in the future would get you more memory in an even faster configuration. If price is important, then it's a dual- board and four sticks.
It's very minor and won't be noticed outside of benchmarks, but if you use both banks of memory your memory controller has to use slightly looser timings to allow the signals to stabilize properly. – Dan is Fiddling by Firelight – 2010-03-16T15:35:28.680
+1 for 'get anywhere near' - you only have enough RAM when you can't imagine using it all. – Phoshi – 2009-12-19T13:09:00.470