In purely gaming scenarios 16gb in dual channel is more than adequate however.

As Wardog stated, video editing sees significant performance improvements with higher memory bandwidth but again, if speed is of little concern to you single channel will work well enough. That said 16gb in dual channel will be faster than 20gb in single channel hands down. While video editing does like more vs faster in a given scenario this is in reference to frequency vs capacity not dual vs single channel. In other words 32gb of DDR3 1600 would typically perform better in large tasks than 16gb of DDR3 1866 or 2133 but, 20gb in single channel would negate any benefit over 16gb in dual channel. Additionally, smaller video editing tasks see even less benefit from the extra RAM, something like a 5min youtube video would likely not be able to access more than 16gb of RAM unless it is in 4k with multiple audio streams etc.

So in summation, 16gb is more than enough for most tasks today and anything more would only be useful in a professional environment like a programming workstation or server for example. 20gb in single channel does not offer enough additional RAM over 16gb to compensate for the loss of bandwidth. Now 16gb vs 24gb in single channel may be worth it in certain scenarios but again, these scenarios would not apply to you as a gamer and sometimes video editor, at least not at the time of my posting this.

If you have 8gb of RAM and an application is using more than 8gb of RAM the system begins swapping to the hard disk. As fast as SSDs may be they are still infinitely slower than RAM so the result, even using a fast SSD, is sluggishness and significant slow down. 

Now games tend to be optimized to utilize memory efficiently as well as keep as much data in memory as possible to cut down loading times. The advantage of this is that most of the data held in memory when gaming is not needed in the moment and some of it is barely needed at all so a properly optimized game will swap less used/critical-data to the hard disk and load it back into RAM when needed. The result is significantly less slowdown/stutter. Even with all this optimization however you can have so little RAM that the game is forced to swap critical/common-use data to the hard disk. It is only in that scenario that more RAM will show any kind of benefit in a gaming system. A particularly well optimized game will try to use as much RAM as it can to speed up loading times but actual gameplay and fps will not be any different between say an 16gb system and a 32gb system for the most part.

Now non gaming tasks work a lot different. Something like batch 3d rendering in an application like 3d Studio Max for example will slow down significantly with less RAM. In this case 20-24gb of single channel RAM will provide a performance benefit over 16gb of dual channel RAM as even at half the bandwidth it is still significantly faster than swapping to the hard disk/SSD. The same applies to program compiling and large video encoding/editing tasks. 

In all situations more/faster memory is always king but when talking more/slower vs less/faster the description above applies.

I hope this little write up is of some help. When determining how much RAM I want in a new build, these are the considerations I make.

Basically if you look at RAM usage in the system resource monitor while performing your every day tasks and see less that 20% available (free or in standby mode) RAM then you could benefit from more RAM.

The above screenshot was taken with nothing but a browser and steam loaded on the system. When gaming the used RAM increases to about 6gb in most titles.

As you can see I have 0MB free and 5gb in standby. 8gb in this instance is enough for most of the tasks I perform on this system. If I install another 8gb of RAM I see the free number increase to arround 5gb with the remainder being either in use or on standby. I ideally you want to see at least some free RAM but different OS versions will reserve, or put in standby, more or less RAM. This is on Windows 8.1, with windows 7 more RAM is shown as "free". So long as the tasks you regularly perform do not use more than 80% of your physical RAM you do not need more or at least will see no benefit from it.