It can be done fully internally but it's cramped... You need a reservoir, a pump, waterblocks for CPU and GPU, a radiator, a fan, tubing and fittings, and coolant... And that's a basic setup. There are way too many options available to you for me to give you links to specific stuff, just check out some pc parts sites and they'll have everything you'll need, though you may have to really look around to find the right stuff to make it internal. I've only ever seen a few fully internal water-cooled 360s...
This is one of the few fully internal water-cooled 360s I've seen, and probably the best write up to follow if you want to replicate it...
http://forums.xbox-scene.com/index.php?showtopic=505987