20- and 24-inch Aluminum iMacs (2008 Edition)

Finally, eight months after the aluminum iMac made its debut, Apple has released an update to its flagship consumer desktop model. Under the hood, the new iMacs differ significantly from the original aluminum iMac, and those differences result in speed increases.

Speed bumps

On the outside, nothing has changed, and the iMac is still available in 20- or 24-inch aluminum-encased models. The major changes involve the Intel Core 2 Duo (Penryn) processor now at the heart of the iMac. Penryn speeds start at 2.4GHz (the speed of the previous high-end standard iMac's processor), and include 2.66GHz, 2.8GHz, and 3.06GHz (the 3.06GHz chip is available in the iMac as a build-to-order option). The previous iMac processor speeds were at 2GHz and 2.4GHz, with a 2.8GHz Core 2 Extreme build-to-order option.

However, a slate of under-the-hood improvements have facilitated a performance spike in the standard models that makes them an especially good value for people looking to upgrade. For example, Apple boosted the system bus speed from 800MHz in the previous iMac to 1,066MHz in the new iMac. The system bus plays a major role in communications between the processor and system memory, which can make everyday computing faster. And, speaking of memory, the standard configuration of the 2.66GHz and 2.8GHz iMacs has been raised to 2GB, installed as a pair of 1GB SO-DIMMs rated at 800MHz. The 2.4GHz iMac has only 1GB of 800MHz RAM, but Apple installs it as a single SO-DIMM so you can add more RAM without having to replace the SO-DIMM the system ships with.

The new iMacs use the now-familiar Santa Rosa chip set--the fourth-generation Intel Centrino platform (combining the processor, chip set, and wireless interface)--found in previous iMac models. The next-generation Centrino platform, code-named Montevina, won't be released until later this year.

The hard-drive capacities of the standard configurations haven't changed. The low-end iMac is still at 250GB, while the other models have 320GB hard drives.

Screen and video

Apple hasn't changed the iMac's glossy screen. While the 24-inch iMac has an 8-bit wide-screen TFT active-matrix LCD that displays millions of colors, the 20-inch iMacs use 6-bit wide-screen TFT active-matrix LCDs, achieving millions of colors by dithering. And like the previous 20-inch iMac, the new 20-inch iMacs show strong color and contrast shifts when viewed at an angle instead of straight on.

Apple still uses ATI's Radeon HD 2400 XT with 128MB of RAM in the low-end iMac. Both the 2.66GHz and 2.8GHz iMacs use ATI's Radeon HD 2600 Pro with 256MB of memory. The 3.06GHz build-to-order iMac features a new video card for the iMac line: Nvidia's 512MB GeForce 8800 GS.


Macworld Lab ran the three new standard iMacs through our customary suite of tests to gauge performance improvements on the previous generation. The new 2.8GHz 24-inch iMac showed a 30-point (13 percent) Speedmark improvement on the previous high-end model, a 24-inch 2.4GHz iMac. However, this new high-end iMac posted the same Speedmark score as the previous build-to-order model, a 2.8GHz 24-inch model. The new low-end iMac, a 2.4GHz 20-inch model, had a Speedmark score that was 26 points higher (13 percent faster) than the previous low-end model, a 2GHz 20-inch iMac. Also, the new 2.4GHz 20-inch iMac scored 9 points lower than the older midrange iMac, a 2.4GHz 20-inch model. But as we pointed out in our earlier Macworld Lab benchmark report of the new iMacs, the older 2.4GHz 20-inch iMac that we tested had a larger hard drive and a better graphics card than the new entry-level model, which explains the speed difference.

There's a considerable performance gap between the new US$1,499, 2.66GHz 20-inch iMac and the new $1,199, 2.4GHz 20-inch model. The 2.66GHz model's Speedmark score was 24 points higher than the 2.4GHz model, a 10 percent difference. If you spend the extra money for the 2.66GHz model, you not only get a speed boost, but also more RAM (2GB versus 1GB), a bigger hard drive (320GB versus 250GB), and a better video card (256MB Radeon HD 2600 PRO versus a 128MB Radeon HD 2400 XT). The extra $300 is worth it.

Longtime Mac users remember the days when 3GHz on the desktop seemed like an unobtainable goal-but that was the era of the PowerPC and single-core processors. The 3GHz desktop Mac arrived in the form of the Mac Pro, and now, a 3GHz-or 3.06GHz, to be exact-system is available in the iMac lineup as a build-to-order option. The 3.06GHz iMac's Speedmark score was only 11 points higher than both the old 2.8GHz build-to-order iMac and the new 2.8GHz model, a meager 4 percent. However, the 3.06GHz build-to-order iMac really separates itself from the new 2.8GHz iMac in graphics performance; the 3.06GHz iMac, with its 512MB GeForce 8800 GS graphics card, blasted by the 2.8GHz iMac with its 256MB Radeon HD 2600 Pro by 18.8 frames per second (28 percent) in our Quake 4 test and clocked in at 11 percent faster in the Compressor test.

Macworld's buying advice

The new iMacs are faster than the previous models, but they cost the same. Unfortunately, the one major thing we didn't like about the first-generation aluminum iMac--the poor viewing angle on the 20-inch screen-hasn't changed for the better. Still, our praise of the previous aluminum iMac still applies: The iMac offers enough features for both general consumers and professionals.

The 2.66GHz 20-inch iMac offers a nice blend of price and performance; it's a better value than the 2.4GHz iMac. The 2.8GHz iMac, in all its 24-inch glory, is a stunning machine and is bound to make an impression in your home or office.

[Roman Loyola is a Macworld Senior Editor.]

This story, "20- and 24-inch Aluminum iMacs (2008 Edition)" was originally published by Macworld.

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