You should probably check with someone else, but I think a 850W will be fine for 780 SLI and a decent overclock.
As far as cases though, you just need to make sure it has mounting for a 240mm or 280mm radiator. If you decide to go with a full-sized closed loop water cooler I have to recommend the Corsair Air540, which is easily the most spacious and user friendly case I've ever owned. It was designed for water-cooling enthusiasts, but it doesn't have the extra features you'll get out of a decent full-sized tower, such as fan switches, LED lights, or abundent 3.5" HDD bays and external drive bays.
I try and pitch it to everyone who asks for a case recommendation. The full-sized window is like a museum display case, with your components as the artwork. But hey, it's all about whatever suits your needs.
Well if it isn't fat stinking billygoat Billyboy in poison. How art thou, thou globby bottle of cheap stinking chip-oil?