Verdict: Still not very competitive, but shows that two-GPU CrossFire has potential.

ATI's first dual-GPU graphics card arrived long before Nvidia's GeForce 7950 GX2, with the launch of the Rage Fury Maxx way back in 1999. Now, though, both companies are relying on dual-GPU cards to form their finest and fastest products.

The Radeon HD 3870 X2 launched before the GeForce 9800 GX2 and, unlike its rival, mounts both of its GPUs on a single, 266mm-long PCB. As its name suggests, it sports two HD 3870 GPUs, each with 320 stream processors clocked at 825MHz - 50MHz higher than those of most HD 3870s. Each GPU is equipped with its own 512MB bank of GDDR4 memory clocked at 900MHz (1.8GHz effective), somewhat slower than the 1.125GHz (2.25GHz effective) RAM of the HD 3870. Each bank of RAM communicates with its GPU via a 256-bit memory controller. This means that each GPU is provided with only 57.6GB/sec of memory bandwidth, much less than the 64GB/sec of each GPU on the 9800 GX2.

The two GPUs communicate with each other via a PCI-E bridge chip, so the HD 3870 X2 will work in any motherboard with a 16x PCI-E slot. The mammoth PCB is powered through 6-pin and 8-pin PCI-E sockets and cooled by an enormous copper heatsink and fan. Unlike the 9800 GX2's cooler, it exhausts heat through the back of the case, but it's still pretty noisy.

The HD 3870 X2 has had rather a chequered history, with early drivers proving unable to unlock the full potential of both GPUs. Fortunately, the latest Catalyst 8.4 driver appears to have resolved many of these issues, with the result that the HD 3870 X2 now performs much better in many games. On the whole, the HD 3870 X2 provides very similar performance to a pair of HD 3870 cards running together in CrossFire. Unfortunately, the HD 3000-series architecture struggles to process anti-aliasing effects efficiently. This, coupled with its comparatively low memory bandwidth, means that the HD 3870 X2 failed to deliver smooth frame rates in many of our test games, including Crysis, Call of Duty 4, ArmA and Need for Speed: Pro Street. As that's four out of ten games, we can't really recommend the HD 3870 X2, despite the fact that it costs a good £80 less than the 9800 GX2.

Three-card crossfire

Each HD 3870 X2 has a single CrossFire bridge connector, so only two cards can be linked together in a CrossFire-compatible motherboard. However, you will need a monstrous PSU with two 6-pin and two 8-pin PCI-E power connectors to power a pair of HD 3870 X2 cards - our test rig drew a ludicrous 690W from the wall when both cards were running flat out.

Unfortunately, very few games take advantage of all four GPUs in this £540 pair of graphics cards. We spent longer testing the pair of HD 3870 X2s than any other multi-GPU graphics system, but even after a week of phone calls and emails to ATI, we were unable to persuade any games other than Crysis, Team Fortress 2 and DiRT to run significantly faster with two cards. Although this is disappointing, ATI was very interested in getting to the root of the problem, so it built an identical Skulltrail system in its Canadian HQ. Although ATI was unable to work out the cause of the poor performance, it did get exactly the same results as Custom PC, so it's definitely not a fault in our test rig, but rather the HD 3870 X2 or its drivers.

What's particularly disappointing is that even when games aren't accelerated by the second card, it still consumes a massive amount of power, not only contributing to your electricity bill but also creating a huge amount of heat that must then be disipated from your PC case.

Even when the second HD 3870 X2 was working properly the gains weren't particularly useful - the minimum frame rate in Crysis did rise by 300 per cent at 1,920 x 1,200, but as this was merely a rise from 2fps to 6fps, the game was still completely unplayable with four GPUs.

Team Fortress 2 showed more of a speed increase, but as the single HD 3870 X2 pushed the game to a stupidly fast minimum frame rate of 79fps at 1,920 x 1,200, it's hard to see the point of buying a second HD 3870 X2 for it. However, the second HD 3870 X2 provided a tangible benefit to DiRT, allowing us to ramp up the resolution from 1,680 x 1,050 to 1,920 x 1,200 while still maintaining a smooth minimum frame rate of 34fps.

Conclusion

A single HD 3870 X2 isn't all that desirable, as it's £70 more expensive and less power efficient than the faster 9800 GTX. The real tragedy of this card, however, is that it doesn't provide better performance, despite CrossFire appearing to have fewer stability and driver problems than SLI. Part of this can no doubt be attributed to ATI's more regular driver updates (at the time of testing, Nvidia hadn't updated the GeForce 8-series driver since December 2007), but we also suspect that the CrossFire driver has less of a CPU overhead than SLI, as none of the strategy games slowed down on the HD 3870 X2 as much as they did on the pair of 9800 GX2s.

Although you'd be crazy to buy a pair of HD 3870 X2 cards, if ATI's next-generation GPU can solve the poor performance of the HD 3000-series GPUs and maintain CrossFire's stability and flexibility, the company could on to a real winner.

• Cases - 21st November 2008
• Motherboards - 24th October 2008
• Gaming Headsets - 26th September 2008
• Dream PCs - 24th September 2008
• Headphones - 12th September 2008
• Media PC components - 22nd August 2008
• PSUs - 14th July 2008

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