Verdict: Reasonably effective in a two-card configuration, but pretty pointless with three or four cards.

The Radeon HD 3870 shares the same basic GPU architecture as the HD 3850, which was based on the lacklustre HD 2900 XT from early 2007. Both the HD 3870 and HD 3850 have a bank of 320 stream processors fed by a 256-bit memory controller. The only real differences between the two cards is that the HD 3870 is clocked significantly higher and has 512MB of GDDR4, while the HD 3850 runs slower and usually has 256MB of GDDR3. First-generation HD 3870 cards were fitted with dual-slot coolers, but newer cards such as the Sapphire Toxic (pictured) have a more compact single-slot cooler. Since its launch a few months ago, the price of the HD 3870 has trickled down from around £150 to £130.

The HD 3870 is one of the few GPUs to support Shader Model 4.1 and DirectX 10.1, although support is far from widespread. Typically, the GPU core and stream processors are both clocked at 775MHz, while the RAM runs at a heady 1.125GHz (2.25GHz effective). Despite this, the HD 3870 has only a single 6-pin PCI-E power socket and is relatively quiet.

The single HD 3870 failed to produce a smooth frame rate (25fps or more) in most of our test games, with graphically intensive titles such as Crysis bringing the card to its knees. In fact, the single HD 3870 was able to achieve a playable frame rate in only three of our ten test games - Company of Heroes: Opposing Fronts at 1,680 x 1,050, and Oblivion and Team Fortress 2 at both 1,680 x 1050 and 1,920 x 1,200.

Two-card crossfire

Many motherboards support two-card CrossFire, as ATI allows Intel chipsets to run it as well as those from its parent company, AMD. It's easy to configure, as you simply install a single CrossFire bridge connector to the sockets nearest the DVI ports. Two Radeon HD 3870s provided the single biggest performance boost of any CrossFire setup, with minimum frame rates in our selection of first-person shooters rising, on average, by 58 per cent at 1,680 x 1,050 and 64 per cent at 1,920 x 1,200. Driving games were accelerated too, becoming 35 and 43 per cent faster.

Unfortunately, this still wasn't enough of a boost to make Crysis or ArmA playable, as Crysis failed to be moved by two cards and ArmA even saw a slight decline at 1,680 x 1,050. It did, however, make S.T.A.L.K.E.R. and Colin McRae: DiRT smooth and enjoyable. Neither Supreme Commander nor Company of Heroes: Opposing Fronts received much of a boost to their minimum frame rates from the second HD 3870, but at least they weren't any slower. Average frame rates rose by 61 per cent in Supreme Commander and 18 per cent in Company of Heroes.

Three-card crossfire

In theory, it's possible to build a three-card CrossFire system with a mixture of HD 3870s, or a single HD 3870 X2 and an HD 3870, but we'd be hard-pushed to recommend either configuration. This is because although three-card CrossFire is faster than two-card CrossFire, any multi-GPU system, be it CrossFire or SLI, produces distinctly diminishing returns over two GPUs.

For example, although two-card CrossFire made several games playable that weren't smooth on a single card, three-card CrossFire was far less effective - its most notable achievement was dragging the minimum frame rate in Crysis into double figures, although the game still chugged. It improved other games, but in general only added to a frame rate that was already sufficiently smooth with two cards.

Four-card crossfire

Very few motherboards are capable of running four HD 3870s together in CrossFire, but given the tiny performance increase over a three-card CrossFire system, that hardly seems important. For an additional 108W of power consumption our four-card HD 3870 system was between 6 per cent slower and 8 per cent faster in all games, depending on the resolution. This is a very similar pattern to the results achieved by the SLI configurations with more than two GPUs, so clearly both ATI and Nvidia have a huge amount of work to do on their multi-GPU drivers. Given the £520 cost, there are much better ways to spend your cash.

Conclusion

While a single HD 3870 struggles to play games smoothly at 1,680 x 1,050, two cards running together in CrossFire provide a far more enjoyable gaming experience. Now that slightly cheaper single-slot, quiet-running HD 3870 cards are available, it could well be worth adding a second HD 3870 to your existing system.

However, this should be balanced against the cost - it's certainly not worth buying a third or fourth HD 3870, for example - the extra money would be far better spent on a more powerful single-GPU card, or elsewhere on your PC, for that matter. Finally, if you're upgrading an older system, a single-GPU card such as a 9800 GTX or 512MB 8800 GTSwill be faster, cheaper and more hassle-free than a pair of HD 3870s.

• 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

• Intel DG45FC - Motherboards
• Intel X25-M Mainstream SATA SSD - Hard disks / storage
• OCZ Core Series 30GB - Hard disks / storage
• Western Digital Caviar Black 1TB - Hard disks / storage
• Asus EAH4870 TOP - Graphics cards
• Creative Sound Blaster X-Fi Titanium Fatal1ty Pro - Sound cards
• Multiwinia - Games