The AMD Radeon HD 6990 is a beast. Soundly outpacing the best that Nvidia has to offer (at the moment), this dual-GPU juggernaut, previously codenamed "Antilles", clambers to the top of our graphics card heap by serving up the most raw power we've seen to date. But then there's the price: at £550, it's strictly aimed at those ultra-enthusiasts who'll spare no expense on their gaming rig.

But rest assured, you're arguably getting your money's worth, provided you're already living luxuriously on the bleeding edge. The AMD Radeon HD 6990 is pricey, but it proved to be demonstrably faster than its closest competitor, the £400 nVidia GeForce GTX 580. For comparison's sake, I also included the £305 AMD Radeon HD 6970, the company's recent single-GPU speed champ.

Inside a Juggernaut

The "Cayman" GPU debuted in AMD's 6900 series, in the aforementioned Radeon HD 6970 graphics card. The Radeon HD 6990 combines a pair of these GPUs onto a single card, think Crossfire on a stick. The end result is a powerful graphics card that fits comfortably into a single PCI slot (though the card is two slots wide), leaving plenty of room for a second...

In keeping with AMD tradition, the Radeon HD 6990 manages to eke out serious speed while sipping electricity. It's designed to meet a 300W TDP at its stock, factory settings. That's relatively meagre for such a beefy part. The card requires dual 8-pin connectors, so make sure your power supply is up to snuff before you grab your wallet.

The Radeon HD 6990 also offers up something special for customers who want to grind the most power out of their bauble. A dual-BIOS switch on the side of the card toggles the part from its stock, factory-supported position to an overdrive mode, with increased clock speed and voltage demand. AMD calls it the "Antilles Unlocking Switch for Uber Mode," or AUSUM. I won't.

Coupled with AMD's Power Tune application, you'll have a wide range of control over the GPU and Memory clocks. For these tests, I also compared the Radeon HD 6990 at its stock speeds (830 MHz), to it's base overclocked setting (880 MHz), referred to as Radeon HD 6990 OC for simplicity's sake.

Before we talk results, lets talk features. The Radeon HD 6990 offers five display outputs: four mini-display ports, and a dual-link DVI port. Every card will ship with 3 adapters (two mini-DisplayPort to DVI, and one mini-DisplayPort to HDMI), for use in an Eyefinity setup. You can drive up to six displays from the card, including arranging five 24-inch screens in portrait mode.

Or my personal favourite: driving three 30-inch monitors. If you happen to own, or are considering purchasing three 30-inch displays, a £500 graphics card is likely pocket change (and if you happen to be looking for a roomate, I'm available).

Crunching the numbers

On to the numbers! Our testbed consisted of a Core i7-2600 processor, running at its stock 3.4GHz clock speed, 4GB of RAM and Windows 7 Home Premium (64-bit). The latest drivers were used for both cards, in AMD's case, it was a preview version of their Catalyst 11.4 driver.

As always, we'll start with the synthetic benchmarks. First up is Futuremark's 3DMark 11. It's the latest version of the tried and true 3DMark benchmark suite, though 3DMark 11 has been designed for the DirectX 11 era. The benchmark churns through a number of graphics and CPU intensive tests, and then assigns a score based on how the hardware performs.

3D Mark 11 Radeon HD 6990 Radeon HD 6990 OC GeForce GTX 580 Radeon HD 6970
Performance 9096 9391 5455 5389
GPU Score 9644 10105 5086 5031
Extreme 3286 3427 1928 1784
GPU Score 3038 3176 1736 1606

The numbers speak for themselves. The Radeon HD 6990 boasts a 67% lead over the Nvidia GTX 580 on the Performance setting, sliding up to 70% in Extreme mode.

The 3DMark 11 score includes CPU tests, so I went ahead and isolated the results for the GPU tests. That's an 89% lead over the GTX 580 on the Performance setting, and a 75% lead at the Extreme setting. In a word: Wow.

Unigine Heaven 2.5 Radeon HD 6990 Radeon HD 6990 OC GeForce GTX 580 Radeon HD 6970
1920x1200 73.6 75.9 48.9 39.7
1920x1200 4xAA 58.6 60.8 40.1 31.6
2560x1600 45.8 47.1 29.5 24.7
2560x1600 4xAA 36.3 37.5 24.5 19.7

The tale is rather similar with our second synthetic test, Unigine's Heaven benchmark. Heaven is a rather forward looking benchmark, gorgeous and graphically intensive in equal measure. If you'd like to try it out for yourself, there's a free version available at Unigine's website.

Performance is a bit closer here, the GTX 580 comes within spitting distance of the 6990, just under 12 frames per second slower at the maximum resolution, 4x AA. The divide grows whenever anti-aliasing is shut off, but you probably aren't going to buy a £500 graphics card to scrimp on visual extras.