Your memory slots are actually in pairs, so as long as the memory in the pairs match, you'll be fine for dual channel operations. Yes, you could install the 2x8 kit and get a total of 24GB of dual channel memory as long as the DIMMs are in the matched slots.
Your motherboard manual will have the info in it.
Regarding buying multiple, single DIMM packages, as long as they are the same model (using the same chipset, etc), they will work as dual channel memory as well.
awsome, thank you