The ATI Radeon HD 4870 X2, AMD's latest dual-CPU 3D card

Two graphics chips. One card. And a whole lotta rendering fun: That's the basic philosophy behind the ATI Radeon HD 4870 X2, AMD's latest dual-CPU 3D card. Actually, that's the philosophy AMD has adopted for high-end graphics in general. No longer intending to duke it out with NVIDIA at the very top of the market, instead of producing one monstrous GPU, AMD's plan is to slap NVIDIA silly with two slightly more compact GPUs crammed onto a single card.

But there's a problem. In the past, two chips on one board has been a recipe for double trouble, not double your fun. Multi-GPU technology in multi-card form has always been dubious in terms of stability and reliability; a somewhat worrisome fact for a single card that relies on multi-GPU performance scaling for its very existence.

NVIDIA kicked off the modern day version of the twin-chip trend with GeForce 7950 GX2. That board worked fairly well at first. But then NVIDIA launched a whole new graphics architecture and, quite frankly, its driver team stopped caring about the 7950 GX2.

And drivers are absolutely crucial when it comes to any form of multi-GPU technology. Both NVIDIA's SLI platform and ATI's competing CrossFire technology use driver profiles to detect games and apply correct multi-GPU scaling methods.

If there's no driver profile for a given game or if the game is not detected correctly, you're in trouble. At best, you'll get single-GPU performance. At worst, the game won't run at all.

The task for the new Radeon HD 4870 X2 is therefore clear. To be taken seriously it must expunge all hint of multi-GPU unreliability and deliver the sort of stability and ease of use that users expect from a single-chip graphics card.

On paper, there are reasons to be optimistic. For starters, AMD has upgraded its inter-GPU PCI Express bridge chip from 1.0 to 2.0 spec and added a 5GB bi-directional sideport to each GPU. Consequently, overall inter-GPU bandwidth has been boosted from 6.8Gbps to 2l.8Gbps.

Likewise, AMD has made a really smart move in stuffing fully 2GB of graphics memory onto this card. At really high resolutions, 512MB per GPU may not be enough to store all the game data. When that happens, a graphics card is forced to use the PCI Express bus to fetch data. And that means hideously slow frame rates. But with 1GB per GPU, the 4870 should suffer from no such shortcoming.

Fully Automagic

The 4870 X2 is also extremely user-friendly by multi-GPU standards. It shows up as single device for driver installation purposes and there's no need for users to even think about enabling multi-GPU scaling it all happens automatically. Unlike most other multi-GPU boards, the 4870 X2 is also capable of full multi-display support.

But, what the 4870 X2 conspicuously does not do is to move the game forward in a technological sense. One day, a company will produce a multi-chip solution that behaves likes a single rendering device, thereby sidestepping current multi-GPU problems. But that day has not arrived. In essence, the 4870 X2 remains CrossFire on a card and is every bit as dependent on driver support as any other multi-GPU technology currently available.

The CPUs are essentially lifted from the single-chip 4870 board. It's the same 55nm GPU die running at an identical 750MHz. The 4870 X2 therefore packs 1,600 stream processor and a theoretical maximum compute performance of 2.4TFLOPs. One helluva lot, in other words.

Likewise, the 3.6Mbps data rate of the GDDR5 graphics memory is identical to the single-chip 4870. And like every other 4800 series card, the 4870 X2 offers DirectX 10.1 support. For what it's worth, NVIDIA's competing GPUs remain 10.0 bound.

Broken down

When the 4870 X2 performs, it works extremely well. Given that the single 4870 is not all that far behind NVIDIA's GeForce GTX 280, it comes as no surprise to find the 4870 X2 has its measure when multi-GPU scaling is in full flow.

The 4870's performance in GRID also shows the benefits of all that video memory. The pair of 4870s in CrossFire mode really fall off a cliff at 2,560 x 1,600. No such problem for the 4870 X2 and its twin 1GB memory buffers. If memory availability is not an issue, the X2 scales largely identically to the 4870s in CrossFire.

That's the good news. Now brace yourself for the bad. The X2 fails to deliver in the one game where you really want maximum performance, Crysis - as do the 4870s in CrossFire mode. Both setups simply crash approximately five seconds after level loading.
Now, we suspect the X2 works as intended in 95 per cent of system configurations. But not, unfortunately, ours. Given more time and correspondence with AMD, we are sure that a solution will present itself.

We really want to like the new X2. In many ways, especially the memory buffers and multi-monitor support, the HD 4870 is a dual-GPU done right. And when it does work properly, it gets immensely fast results. But the harsh truth is that as long as multi-GPU technology relies on driver profiles, it will be a flaky, hit-and-miss affair. And one that we can't in all conscience recommend that you buy.