6 best Crossfire and SLI graphics cards on test

Can you have too much of a good thing? That's the first question that leaps to mind in the context of these ridiculous dual-card, quad-GPU graphics solutions. Well, that and the one about your sanity for even considering such wanton decadence.

For starters, the raw specifications of both the ATI Radeon 4870 X2 CrossFireX and Nvidia's competing GeForce GTX 295 SLI are almost too much to comprehend.

In no particular order, the edited highlights include a grand total of nearly 8GB of graphics memory shared between all four cards, getting on for a terabyte per second of memory bandwidth and 2,560 stream shader cores. Madness.

Then there's more madness: the cost. You won't get much change out of £700 for a pair of Radeon HD 4870 X2 boards. Hard to believe, but the GeForce GTX 295 duo is even worse at slightly under £900. Unless you're a Westminster MP bagging goodies on the tax payer's ticket, that's pretty hard to swallow.

These cards are monstrous physical specimens, too, immensely long and enormously heavy. Ultimately, the very concept of cost has to be excluded from the equation if the 4870 X2 CrossfireX and GeForce GTX 295 SLI are to make any sense at all.

For that matter, given how much power these beastly boards consume you'd better not give much thought to the livelihood of birds, bees and lovely old trees, either.

With four high-end GPUs in your PC and you'll be looking at a system that guzzles around 700 watts and that doesn't even include a monitor or speakers. With current concerns regarding the environment in mind, that's probably downright immoral.

And yet at the same time the mad scientist in us can't quite resist the lure of the most powerful graphics solutions available to humanity. Who wouldn't cackle with delight and cry out "It's alive!" as four monster GPUs spool up? Okay, maybe it's just us…

The problem is, the laughter quickly turns to tears when you inspect the benchmark results. But before we come to the detailed performance analysis, let's remind ourselves what makes these beasties tick. The ATI Radeon HD 4870 X2 has been around for some time and is based on a pair of AMD's enormously successful RV770 GPUs.

Key specs include 1,600 stream shaders per card, a core clockspeed of 750MHz and 1GB of GDDR5 memory running at an effective rate of 3.6GHz.

In total, each board packs nearly two billion GPU transistors and a theoretical maximum computational capability of 2.6 TFLOPs.

No compromises?

Needless to say, you can double most of those figures for a pair of 4870 X2s running in quad-GPU CrossfireX mode. But perhaps the best thing about the 4870 X2 is that it brings with it absolutely no compromises compared with the single-GPU card upon which it is based. It has the same clockspeeds and memory buffer as the Radeon HD 4870.

That means that when CrossFire mode doesn't work, you can at least be confident of getting the best single-GPU performance that AMD can offer. Or at least that used to be the case until AMD released the slightly upgraded Radeon HD 4890. So it goes.

As for the GeForce GTX 295, it follows a slightly more typical path for a multi-GPU graphics card in that some compromises have been made. It's based on Nvidia's epic, 1.4 billion-transistor GT200 GPU. But such is the heinous power hungriness of that chip, Nvidia had to give it a bit of a chop here and there. Mercifully, that doesn't include the shader array.

All 240 units are present and enabled in both GPUs which translates into 480 per card and 960 in quad-GPU SLI configuration.

Likewise, all 80 texture units make an appearance. Take a peek at the clockspeeds, however, and you begin to see where Nvidia has cut corners. Both the core clockspeed of 576MHz and the shader clock of 1,242MHz are well down on those of the fastest single-GPU GT200 board, the GeForce GTX 285.

What's more, Nvidia has taken the knife to GT200's render outputs, reducing the number from 32 to 28 per GPU core. That in turn has a knock-on effect on the memory bus and frame buffer. The former shrinks from 512-bit to 448-bit, while the latter drops from 1GB to 896MB, again per GPU.

All of which means that in the unfortunate event that multi-GPU scaling fails to work, systems based on both the Radeon HD 4870 X2 CrossFireX and the GeForce GTX 295 SLI will fail to match the performance of their closest single-GPU relatives.

That's a sobering thought when you think about how much a pair of these cards cost. The good news is that in our benchmarks, there's no evidence of either CrossFireX or SLI failing to provide at least some multi-GPU scaling.

Both are significantly faster across the board than the best single-GPU cards currently on the market. However, when you factor in the competition from the fastest dual-card solutions – a pair of Radeon HD 4890s from AMD or two Nvidia GeForce GTX 285s – the wheels begin to come off.

In the case of the Nvidia comparison, the quad-GPU setup delivers very little extra performance in World of Conflict and is actually significantly slower in Far Cry 2, even if it does take an easy victory in Crysis Warhead at the enthusiastcentric 2,560 x 1,600 resolution.

In the AMD camp, forking out for quad-GPU kit will buy you moderate performance gains in the region of 20 to 25 per cent in Far Cry 2 and World of Conflict, but you'll have to swallow a performance penalty of a similar magnitude in Crysis Warhead.

Is quad-GPU therefore a step too far? We think so.