Two 4GB 680's for a single 1080p screen?
Might want to re-think your build, that is very overkill.
But your here for advice on water, so I'l run with it.
First off, you want to read the watercooling sticky, it will tell you a lot of the information you need for water-cooling and will give you the means to research the more advanced or nitty gritty stuff. http://www.tomshardware.com/forum/277130-29-read-first-...
Once you'v read through that, post back with any questions you have.
Water-cooling is one of the few places where we wont just give you a shopping list to go buy, its much better if you understand the reasons why the components are picked. Because ultimately, it will be you who is going to be building, maintaining and upgrading this thing.
Were here to make your first time water-cooling a smooth experience, but not to do it for you.