1) 650 watts is pushing it if you're going to do crossfire; and IMO really not enough if you're going to over clock said cross fire.
But for one gpu, it should be fine
2) depends on the manufacturer i believe. So read up before you purchase
3) you need to go read; usually it's the built in overclocking and cooling controls and the fancy buttons on the mobo that varry if they're the same chipset.
4)H70 works fantastic, (corsair i believe) that whole series of cooling products work really well (except the tiny H40 or what ever it is) If you want the beast they just came out with a H100 in the last few months. It's all self contained with a 24 cm radiator with two 120mm fans on it.
It look's pretty cool. (pun intended)
As for a case; I like the cooler master Haf 932 or the haf-x the latter being huge. Almost too big. Corsair makes some cases designed specifically for those self contained water cooling units so look at the propaganda on their site.
IMO haf 932 is perfect for most users.