Thanks for your feedback.
For my second question, how much radiator does it take to cool X watts, I found an interesting graph at skinneelabs
A 3x120mm radiator with 3x120mm fans at 600 RPM can handle 550 W with a 1.5 GPM (gallon per minute) flow rate, if you don't mind a 22 C temperature delta between air and water.
This would put water temperature in the 40-50 C range. I think this is unproblematic for graphics, but I would probably want it lower for CPU and overclocking.
An interesting test of pump noise at martinsliquidlab
says that the best pumps, when fully decoupled, can deliver 1.6 GPM with about the same noise level as a Gentle Typhoon at 700 RPM.
High-end single graphics cards (GTX 580, Radeon 6970/7970) are around 250 W TDP.
Dual cards (Radeon 6990, GTX 590) are around 400 W TDP.
So one 3x120mm radiator can handle two single cards (500 W) or one dual card (400 W) with the noise level of four 600-700 RPM fans.
That doesn't sound bad at all, and a triple radiator will even fit in some full-tower cases.
Four single cards (1000 W) or two dual cards (800 W) would require two triple radiators.
At that point I guess you need external radiators, perhaps something like this
So as a tentative rule of thumb, for "lowest possible noise with no or modest overclocking", I'd go with
* Up to 350W/single graphics: Air
* Up to 500W/dual graphics: Internal 3x120mm radiator
* More than 500W: Exernal radiators
This is based on a couple of days of reading up on watercooling, feel free to disagree