"I'm going to be editing .................It's HD footage which is 24 mbps and 1920 x 1080.......... 24GB ram doesn't cost that much more than 16GB....."
If you're going to be editing footage where just one second is three megabytes in size then you'd probably see the benefit of adding 32GB of RAM which is the maximum your motherboard can support.
Your motherboard supports dual-channel memory which means for optimum performance modules should be installed in matching pairs. Currently you've got one matched pair and one single module. This is another good reason to order four 8GB modules instead of three.