If it were me I'd just get 5 more sticks of the one you already have, save yourself the ~$300. With 24GB you'll have to have a chip with a great IMC to get much OC out of those sticks either way.

As for ECC, I've been hearing "must get ECC!" since people were rolling 8GB back when 2GB was the norm Seems like the "ECC required above xxGB" argument scales linear to the average amount of system RAM, when you go above and beyond the norm I guess.

I don't foresee any issues with 24GB as long as they are stable, if you are just doing video editing and VM stuff (assuming you have storage off your OS drive) it won't be the end of the world if you get an error once in a blue moon.