iMac 24-inch full review

Finally, eight months after the aluminium 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 aluminium models – mainly in that they are much faster. On the outside, nothing has changed, and the iMac is still available in 20- or 24in aluminium-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.

Speed bumps

However, a slate of under-the-hood improvements have led to a performance spike in the standard models that makes them 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 versions. 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, codenamed 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.

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

Card shark

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.

The graphics card found in the top-end 24in iMac is making waves. NVIDIA's 8800 is part of the GeForce 8 series of graphics cards. These cards are ideal for playing the most demanding 3D games; plus working with high-end imaging applications such as Aperture 2.0 and 3D apps.

The card itself comes in several different flavours: GS (used in the iMac); GT (used in the Mac Pro) and there are also GTX, GTS and Ultra cards available for PC users.

The GT and GS cards sit in the middle of NVIDIA's 8-series range, and feature slightly different clock speeds (600MHz on the GT, 550MHz on the GS.)

However, both of the cards used by Apple possess 512MB of RAM, which has raised a few eyebrows because NVIDIA's 8800 GS card is only supposed to have 384MB of RAM (you can see the full spec list at ).

It actually transpires that the 8800 GS in the iMac isn't actually an 8800 GS at all, but the same 8800 GT card from the Mac Pro that has been clocked down to match the 8800 GS speed. This is great news because it means the Memory Interface is 256-bit, rather than 192-bit; plus you get the extra memory itself. On the whole it should sit somewhere between the 8800 GT and a regular 8800 GS. We assume that the card had to be restricted because it would overheat in the small form-factor of the iMac. The 8800 card itself is fairly large (hence it's absence from the 20-inch iMac models) and runs at considerable temperature.

It may even be possible to overclock the card in the iMac to match the full speed of the Mac Pro card, although we wouldn't recommend it because the excessive heat would almost certainly cause your iMac to crash.

Mind the gap

The 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 24in iMac showed a 30-point (13 per cent) Speedmark improvement on the previous high-end model, a 24in 2.4GHz iMac. However, this new high-end iMac posted the same Speedmark score as the previous build-to-order model, a 2.8GHz 24in model.

The new low-end iMac, a 2.4GHz 20in model, had a Speedmark score that was 26 points higher (13 per cent faster) than the previous low-end model, a 2GHz 20in iMac. Also, the new 2.4GHz 20in iMac scored nine points lower than the older midrange iMac, a 2.4GHz 20in model. But as we pointed out in our earlier Macworld Lab benchmark report of the new iMacs, the older 2.4GHz 20in 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 £949, 2.66GHz 20in iMac and the new £749, 2.4GHz 20in model. The 2.66GHz model’s Speedmark score was 24 points higher than the 2.4GHz model, a 10 per cent 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 £150 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 line-up 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 meagre four per cent. 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 per cent) in our Quake 4 test and clocked in at 11 per cent faster in the Compressor test.

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