I hope you've been reading up on the stickies and whatnot.
As for adding a GPU, I don't think you'd have enough rad space to cool it quietly. A thicker 360 with high speed (i.e. high noise) fans could most likely do the job, but if you're looking for a near-silent solution, you should be looking at much more space.
Take a look at some of the Martin Liquid Labs radiator round ups. From there you can get an idea of how much heat the rads will dissipate with specific fan speeds. Then you just need to match the heat dissipation to the heat production (generally the TDP of the CPU + GPU) in order to find the minimum cooling you need.
Please realize that watercooling doesn't automatically mean 'silence'. In fact, my watercooled loop is more noisy than most people's air-cooled rigs I know. When you add radiators, you have your normal case fans IN ADDITION to fans needed for radiator cooling. Simple math = you have several more fans that create noise.
Also, noise level is highly subjective. Silence means many, many different things to many people.