The fastest single-GPU model out of the AMD stable is a tough one to recommend. As the first of its Northern Island codenamed graphics cards, it's certainly the quickest AMD has come out with.

The big problem, though, is the second-tier card released alongside it – the HD 6950. But we'll come to that later.

The Cayman XT at the heart of the HD 6970 is essentially the same GPU as paired in the Antilles chip of the dual-GPU HD 6990. It's clocked faster than the stock specs of the Antilles, though – through the shaky-ground magic of the Antilles Unlocking Switch (also known as the Screw Your Warranty Switch) the HD 6990 can run with its GPUs at the same speed as the HD 6970.

Forgetting warranty-busting overclocking, a pair of HD 6970s will happily outperform the HD 6990. This isn't a huge surprise, given the faster clockspeeds of the single-GPU cards, but it's interesting given the price difference between the CrossFire pairing and dual-GPU card. Effectively, there isn't one.

Two HD 6970s can be found for the same sort of cash you can buy one HD 6990 with. The same power issues ring true for the HD 6970 as for the GTX 580. If you bump up the HD 6990 to the same speeds as the HD 6970 then you're essentially drawing up to 450W from a single source. Should your PSU not be the strongest, you'll soon find yourself in fail city.

With a pair of HD 6970s you'll be drawing just as much power – possibly more. But it will be spread out more evenly and won't be stressing the PSU quite so much. This makes the HD 6990 almost unnecessary, because most motherboards come with a second PCIe x16 lane, specified for CrossFire, as standard. There's little need to pick up an HD 6990 over a pair of HD 6970s.

Benchmarks

DirectX 11 tessellation performance
Heaven 2.5 FPS: Higher is better
Single: 15.5
SLI: 31.3

DirectX 11 gaming performance
Metro 2033 FPS: Higher is better
Single: 13
SLI: 24

DirectX 11 gaming performance
Aliens vs. Predator FPS: Higher is better
Single: 25
SLI: 52

DirectX 11 gaming performance
Lost Planet 2 FPS: Higher is better
Single: 25
SLI: 45

DirectX 11 gaming performance
DiRT 2 FPS: Higher is better
Single: 55
SLI: 99

DirectX 10 gaming performance
Just Cause 2 FPS: Higher is better
Single: 30
SLI: 60

Friendly fire

So far, so good. But what of the HD 6950 we spoke of earlier? The second-tier Northern Island card comes with a Cayman-based GPU, like the HD 6970, but with slightly lower clockspeeds and 128 shader cores turned off. It has the same basic specs, 2GB GDDR5 and a healthy 32 ROPs, and that all gives almost equivalent performance.

The first issue is that it's almost £100 cheaper. The second is that it uses the same Cayman core. Whether it was lazy engineering or an impressive marketing strategy, the HD 6950's GPU wasn't locked at the hardware level.

So by virtue of a simple BIOS flash, made even safer by the dual-BIOS switch, you can effectively turn the HD 6950 into an HD 6970 for free. This makes the HD 6970 a card that nobody really wants, and anyone who's actually paid for one ends up feeling rather sick and/or foolish as a result.

As we said, it's tough to recommend AMD's top single-GPU – it's just too close in performance and too far away in price compared with even the stock HD 6950.

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