The thing about watercooling is that it's an efficient way of moving heat away from GPUs/CPUs/whatever to *somewhere else*... but you're still going to need to dissipate that heat into the room or out of the window, to prevent your stuff from overheating. Regular watercooling uses radiators with fans to get rid of the heat into the room... so you've still got fans and their associated noise to deal with. It's no doubt quieter having a couple of large, slow, quiet fans on a radiator than it is having lots of small noisy fans (e.g. stock CPU heatsinks and GPU coolers). But to get really really quiet watercooling, you just use a huge resevoir/radiator to get rid of the heat outside the case, passively.
Zalman's Reserator series are good for this. I've used one for many years with great success/quietness. There are cheaper roll-your-own options too, but Reserators work well and (IMHO) look great, off the shelf - good for lazy people like me.
In case of using external radiators, all the case needs to provide is two (preferably grommetted) holes to pass through the back/top of it - one for outwards and one for the return flow from your radiator.
After saying all that, I'm probably going to try aircooling for my next rig. I love my Reserator but fear it might not be able to cope with a Core i7 combined with a modern (i.e. hot!) graphics card... and then there's the faff of having a less mobile PC and worrying about leaks during setup etc.