But, in reality, dual channel doesn't matter, as we are in the 4GB zone right now
Whoa! Sorry, but that's not true at all. Having 4Gb or more does not in any way
negate, marginalize or diminish the advantages of dual channel operation. Got a link to any white paper, report, hardware review site, IT news article (anything but another forum poster) to support that? I would really be interesting in reading it.
Dual-channel, in effect, doubles the bandwidth of the memory bus allowing for a "theoretical" doubling of data transfer speeds. Having lots of RAM does not change that, or make that fact less significant or immaterial. Since theory and real-world rarely meet eye-to-eye, you will not realize X2 transfer rates, but you will see faster rates than with single channel. And in this context, the "amount" of RAM has nothing to do it.
Also, many dual channel motherboards with 3 sticks will run the first pair in dual channel and the 3rd stick in single channel. This depends on the motherboard and since we don't know that information, we can only guess. I note many of the newer ASUS and Gigabyte boards (what we use on most of our builds here) operate that way - though pairs are still preferred.
@cravingsleep - note while the packaging may say dual-channel "kit" and single-channel "kit", "kit" is just a marketing and packaging/labeling term. The sticks are all the same. In the old days (dual channel's been around for well over 10 years), RAM makers used to individually test and pair up sticks based on their electrical characteristics. That was a time consuming and expensive process. But not today. Manufacturing techniques have improved significantly such that tolerances are much tighter so individual testing and matching are no longer necessary. Also, memory controllers on motherboards have improved significantly such that they can easily compensate for minor differences - such that many motherboard manuals don't even specify they have to be the same brand any more - though it is usually recommended.
So while they say dual and single (or triple) channel "kits", that is just a packaging and labeling thing. The sticks are identical. It is not a marketing "gimmick" either. Packaging, inventorying, and buying in quantity is almost always better for manufacturers, retailers, and consumers too.
In any case, adding a single stick should cause no problem. However, you must have a 64-bit OS to use that extra 2Gb.
Although getting a little old now I recommend reading Intel Dual-Channel DDR Memory Architecture White Paper
by the RAM makers Kingston.