Your radiator should be used as an exhaust, taking the air inside your case (slightly warmer than ambient) and blowing it through your radiator to the outside
I totally disagree. You want to cool your radiator with the coolest air possible. HDD, CPU(s), RAM and your video card all contribute to make your case (typically) *much* higher than ambient. The whole point of the radiator is to bring the temperature of the liquid down to the temperature of the air you are cooling it with. If your inner case temps are 10° higher than the air sitting outside the case, you are basically saying that at max efficiency, your radiator will be cooling 10° higher than ambient. That sucks.
It is MY opinion that 90% of all water cooling setups should have their radiator setup as an intake rather than an exhaust. Most people put their radiator in the lower front of their cases. This makes great sense because the typical PSU is on the upper rear of the case. Hot air out back, cool air coming in front. If your radiator is blowing hot air out front, and your PSU is ALSO blowing hot air out the back... congrats! You just built your own space heater.
Provide your radiator with the coldest air possible. Bring the air in from the front. The only downside to this is that your radiator will then be heating what could possibly be the main source of incoming air... raising internal case temps. This isn't that big of a negative aspect, as a 4°(unlikely) rise in case temps won't have a noticable difference in anything you are watercooling. Let's say you are simply water-cooling your CPU. Everything else is air-cooled. Your CPU will be putting off MUCH less ambient heat than an air-cooled CPU, so that minute rise in case temps from the radiator is hardly seen. Plus, by having an efficiently cooled radiator (by way of the colder incoming air) your CPU will shed heat into the water better, again decreasing the total case temps. So the 'downside' to pulling in cold-air through your radiator isn't really a downside at all. Your CPU *will* run cooler and it's hard to argue that.
Now, if you are putting waterblocks on SEVERAL components, your cooling potential is even greater. The less heat your components radiate into the case, the better your cooling will be.
In addition, everytime someone sets up a radiator as an exhaust, a fairy dies. Clap clap clap!