As far as I know, every CPU on the market can be cooled more quietly with air than with water.
The same goes for any single graphics card up to and including GTX 580 (by using an aftermarket cooler).

But if we keep adding graphics cards - dual, triple or quad GTX 580 - there will presumably be a point where water cooling becomes more quiet. A forum post suggests that "quiet air cooling" tops out at one high-end graphics card, that's my expectation too.
What are your experiences?

And if I were to water cool dual/triple/quad graphics cards, what kind of setup would be needed to dissipate 500/750/1000 W of heat?

Are there rules of thumb along the lines of "a 2x120mm radiator can handle X watt quietly"?

I would say that the crossover is approximately at a system power consumption of about 350W. I was able to cool an overclocked Pentium-D system quietly with air using giant heat-pipe sinks, quiet fans, and custom ducting. Others with similar power draws also got quiet cooling with water. My system didn't have much headroom; slightly more power would have required significantly faster (noisier) fans. My more recent systems have drawn a lot less power, and have been easy to cool quietly (usually silently) with air.

Recent designs have of course flipped the CPU/GPU power ratio. Recent CPUs, even overclocked, rarely draw more than 100W, while recent GPUs, well, let's just say HOLY COW!

So if you have a single GPU, air is best. If you're going radical hi-def 3D multi-GPU gaming, water might be in your future.

The "crossover-point" is when your air cooling doesnt have enough surface area to remove the heat quietly, and you have to resort to higher fan speeds and more noise to avoid overheating.

E.G.: Multiple high-end graphics cards, highly overclocked quad+ core cpus etc.

The beauty of watercooling is that you can get a very large surface area with big/multiple radiators, enabling you to use low airflow to remove (dissipate) the heat. That, along with quiet water pumps can give you outstanding thermal performance while keeping the noise level very low. This isn't free though, you need to invest in quality gear.

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

In both cases you need airflow sufficient to remove the generated heat. With water cooling you have an additional step of the heat being carried by water to the fins that disperse the heat into the air... and you also have the additional noise of a water pump.

In theory, the water cooling will always be louder. Though as the total amount of heat to be dispersed increases, aircooling with air you are trying to squeeze through a computer case is insufficient and too loud because of the constrictions.

Also, the fans on most video cards are just too loud. It doesn't take much of a water cooling system to cool the same amount of heat much more quietly.