Apple has updated its line of iMac all in one PCs, replacing the old 20in and 24in models with new 21.5in and 27in models featuring a true widescreen 16:9 aspect ratio and impressive high-end features previously found only in the Mac Pro line, including the first quad-core processor to feature in an iMac.

As is common with new Apple products, the new, larger iMacs offer more features than their predecessors, but start at the same £949 base price, with the 3.06GHz model costing £1,119. The new models feature a new enclosure that's wider, to encompass the wider aspect ratio, with an all-aluminium back.

The displays on both models are backlit by LEDs, marking the first time Apple has integrated this bright, instant-on lighting in a desktop computer. The 21.5in display has a resolution of 1920x1080 pixels - the equivalent of a 1080p HDTV - and features 90 percent of the resolution of the old 24in iMac model.

Apple iMac 27-inch 2009

The massive 27in model's resolution is 2,560x1,440 pixels. Both use the IPS display technology used previously only on high-end iMacs, meaning both systems have good colour fidelity and a 178-degree viewing angle without colour shift. The 27in iMac will cost £1,349 in the UK or £1,599 for th 3.06GHz version.

Following the lead of the MacBook Pro line, these new iMac models feature an SD card slot immediately below the optical drive slot on the righthand side. The new iMacs will come with the new version of Apple's wireless keyboard and the new Apple Magic Mouse, which Apple has also announced today. Customers who prefer wired input devices will be able to opt for them as a configure-to-order option from Apple, for the same price.

These new models offer a new RAM ceiling of 16GB, double the maximum memory of previous models. The iMacs can now can be configured with up to 2TB of storage.

The low-end 21.5in iMac model, priced at £949, features a 3.06GHz Intel Core 2 Duo processor, 4GB of RAM, a 500GB hard drive, and nVidia GeForce 9400M video circuitry. A £1,199 model features the same specs other than a 1TB hard drive and the Radeon HD 4670 video card. The £1,349 27in model features the same specs as the £1,199 model, but with the 27in display. All of those models are available immediately.

The £1,599 high-end model, which won't be available until November, brings the iMac product line to new performance heights. It's powered by a 2.66GHz Intel Core i5 quad-core processor and features a Radeon HD 4850 video card. A configure-to-order option will swap in the Intel Core i7 chip for the Core i5.

Based on the Linfield processor that's part of the same Nehalem chip family found in the Mac Pro line, the Core i5 is a four-core processor that features ‘turbo mode', technology that allows the chip to shift automatically from a slower clock-speed, four-core mode into a faster-speed mode with only two cores active. The turbo-mode speed of the Core i5 chip can go as high as 3.2GHz, and the turbo-mode speed of the Core i7 can go up to 3.46GHz.

In a clever touch, all these new iMacs have a new feature that dramatically increases their versatility. Via a series of adapters Apple says will be available shortly, users will be able to attach external display sources, such a DVD players or even other computers, to the iMac's display. At that point, those sources will take over the iMac's display, effectively turning it into an external monitor or an HDTV.

Also announced by Apple today: Updated unibody Apple MacBook.

Also announced by Apple today: a new multi-touch mouse, the Apple Magic Mouse.