Dual- and tri-channel compatibility

You are correct. The Core i5 requires dual channel memory, i.e., 2 or 4 modules. If you get a triple channel kit, then at least one module will be running in single channel mode.

Only if your board had six slots - almost every board has a maximum of 4 slots for RAM.

This totally unnecessary unless You buy top of the range top speed hand tested memory kit for top overclocking scores in witch case You will not be using more then one set anyway.

As long as they are same specifications (speed , timings, voltage) You are fine, preferrably same manufacturer and model as well. I have been mixing and matching memorys with different specs as well without any problems

More than 6 gigs of RAM is slight overkill on an i7 920 system, so I assume 4 gigs on an i5 750 system would be slightly under par. What is the optimal way to purchase 6 gigs of dual-channel memory? Is it possible? Like say if there were 4 sticks and each was 1.5 gigs, that would be 6 gigs and dual channel, and would only fill up 4 slots. Right?

Whether 6GB is overkill has a lot less to do with what processor you're using and a lot more to do with what you're trying to do with it. If your system won't accommodate exactly the amount of memory your want, you're better off to go for the next larger increment rather than the next smaller.

If you really want exactly 6GB in a dual-channel system, and assuming your motherboard has 4 or more DIMM slots, you have two options:

1) Buy a pair of 2GB modules and a pair of 1GB modules, or

2) if your motherboard supports it, buy three 2GB modules and run with two DIMMs in on channel and one in the other.

Some people will tell you that (2) is not a good idea because the third DIMM module won't operate in dual channel mode. This is true, but in real life you're not going to notice any difference in performance. The CPU is actually very insensitive to RAM speeds since its on-chip caches mean that only a very small percentage of instructions ever actually need to access RAM.

With option (2), some people would also warn that you'll won't be able to find a matching DIMM for that 3rd module when it comes time to upgrade. Unless you're overclocking and pushing the RAM to it's limit, that's really not an issue. And, to reiterate the point I made above, overclocking RAM is the least effective way to improve performance when you're tinkering with a system.