Verdict: These once-mighty CPUs aren't worth buying now.

When it first launched in November 2006, we were surprised to see the world's first quad-core CPU, the Core 2 Extreme QX6700, so soon. However, just how groundbreaking the QX6700 was only became apparent when AMD took a further 11 months to launch its first quad-core CPU, the Opteron 2300-series.

Amazingly enough, the QX6700 is still on sale, although it has been superseded by several other Extreme edition CPUs - the two QX6-series models reviewed here and the two QX9-series models reviewed below. All three QX6-series Extreme-edition CPUs have four physical cores, and are essentially two dual-core dies sandwiched together in one package. All the models are made using the same 65nm manufacturing process, and each pair of cores has 4MB of shared Level 2 cache, making a grand total of 8MB for the entire CPU. The QX6700 is clocked at 2.66GHz and has a 1,066MHz effective FSB, the QX6800 at 2.93GHz with a 1,066MHz effective FSB, and the QX6850 at 3GHz with a 1,333MHz effective FSB.

All three CPUs support 64-bit instructions, SpeedStep, Execute Disable and SSE3 and have a fully unlocked multiplier, making them very overclockable.

The QX6700 used to be the fastest desktop CPU in the world, but you can now buy a non-Extreme edition Q6700, which runs at the same frequency for far less. The performance of the QX6800 and QX6850 is harder to match, but both have been superseded by the awesome QX9650 and QX9770. The latter two CPUs offer great performance, thanks to the improved Penryn architecture and larger Level 2 cache. They also run cooler and consume less power, since they're made using a 45nm manufacturing process.

It makes little sense to buy a Core 2 Extreme QX6-series CPU now, although it's worth looking for one on eBay from somebody upgrading to a QX9-series Extreme edition and then clocking the nuts off it, thanks to its unlocked multiplier.

• Gaming Headsets - 26th September 2008
• Dream PCs - 24th September 2008
• Headphones - 12th September 2008
• Media PC components - 22nd August 2008
• PSUs - 14th July 2008
• Multi-GPU graphics - 20th June 2008
• CPU water cooling - 17th June 2008

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• Hyundai W240D-PVA - TFT monitors
• Intel X25-M Mainstream SATA SSD - Hard disks / storage
• S.T.A.L.K.E.R. Clear Sky - Games
• LanCool Metal-Boned K10 - Cases
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