1. Yes, my brother have dual channel motherboard which has 2 ddr2 slots and 2 ddr 3 slots.
2. You can't get but you can make it. Custom built water cooling systems are expensive (about 200 dollars), a little bit hard to make but very effective. But make sure that you really need that.
3. No you don't. Water cooling is just 2 fans and some water circulation, which aren't very power demanding.