Get the single stick at 3000. Once the RAM stability is fixed it should get 2933. Right now it may only get 2667 but the 2 stick will also work at lower speeds possibly 2400. You need to keep the speed up as thats speed the CCX works at.
It depends. If you only have 2 DIMM slots, get the single 8GB and run single channel for now, and add another stick and run dual channel later. But either way, you may only get the RAM to run at 2666 anyway.
Get the single stick at 3000. Once the RAM stability is fixed it should get 2933. Right now it may only get 2667 but the 2 stick will also work at lower speeds possibly 2400. You need to keep the speed up as thats speed the CCX works at.