Our DIY PC with Koolance Exos AL water-cooling kit installed.

Our earlier tests with the Intel Pentium D 805 showed how clock rates of 4 GHz or higher could be achieved without system instability, while a water cooling was required. The good news is that vendors such as Thermaltake, Innovatec, Corsair and others offer water cooling kits at reasonable prices. As far as a good fan-based CPU cooler such as the Zalman CNPS9500 goes, the price tag is at least $50 and is always louder than a good water cooler, the prices of which start at over $100.

When it comes to cooling performance, water-cooling rigs from Innovatek and Koolance achieve very good thermal resistance value of between 0.30 and 0.32 K/W. Cooling fans run at 12 Volts and their sound pressure level readings of 50 dB(A) are not excessively loud. You can quiet things down by lowering fan input voltage to 7 Volts, and get SPL readings of only 44 dB(A). Cooling performance changes very little, and produces thermal resistance in a range from 0.31 to 0.33 K/W. Lowering the voltage still further to 5 Volts reduces cooling performance but also lowers SPL readings to 40 dB(A) as well.

A look at the water cooling block, beneath which the Pentium D 805 is housed. To the immediate right, you'll see a passively-cooled ATI 1600 graphics card, which is adequate for non-gamers.

The Basic Set from Innovatek and other similar offerings from Thermaltake, Zalman and others are available for under $150 and can handle clock rates up to 4 GHz. The Koolance Exos AL offers yet more cooling potential - as you'd expect from a rig that costs nearly $280. This offering delivers more heat exchange benefits, so that those who want to push clock rates beyond 4 GHz are well-advised to make the investment.

The external water cooler can be mounted on top of the case using chain-link type fasteners.

You might actually be able to push the clock to 4 GHz with the Zalman CNPS9500 cooler, but the noise levels at the fan speeds required are unacceptable (if not intolerable). In that case, you're better off running the CPU at 3.6 GHz so as to keep things cool without trading off too much performance against diminished heat output.

It is important to keep the reservoir filled to the full line. The inlet screw is easy to open and close using a coin.

For best results, stick to distilled water (if not some other non-conductive cooling liquid).