Hi
I have noticed that the Corsair Dominators with a higher speed like 2000mhz are becoming a rarer sight in the shops and retailers recently. I dont think you will be able to get any 8GB sticks so from what I've seen so you will have to go for the two 12GB kits. I have just had the same sort of problem as I have upgraded from 6 to 12GB recently. When I first purchased my board there were fewer 4GB sticks around and the norm was 2GB so I bought a 6GB kit.
You shouldn't have any major problems with the upgrade though as they are top quality memory. You might have to tweek the voltage if anything.