CrossfireX and SLI: Platform Choices

For NVIDIA it’s a clearcut set of options when choosing an SLI motherboard; you need an NVIDIA NFORCE motherboard with at least two PCI-e slots. To get the benefit of two x16 slots, you'll need a 780 or 790 board, while the lower level 650/750 chipsets will limit your bandwidth to eight PCI lanes on the second card. That's fine for pairing up a couple of mid-range cards, but will hold back Ultras or GTXs in a more powerful rig.

All cards in a GeForce SLI set-up also have to match - not necessarily by brand name, but by chip class. So GeForce 8800GT will only work with another GeForce 8800GT and so on. AMD on the other hand, has a far more flexible platform. Which is just as well, because if you were restricted to using the firm's Phenom chips there'd be no point in considering CrossFire at all. In our most recent tests with a Phenom 9600, games were hitting the CPU threshold even on a Radeon HD4850 card. Adding more graphics cards made no difference to in game performance whatsoever, because the CPU couldn't keep up.

In what seems like a surprising quirk of nature, though, you can put two or more AMD cards into the current generation of Intel motherboard chipsets, and they'll work just fine. And unlike NVIDIA, you don't need to match cards of the same type, just the same generation. So an HD3850 will work with an HD3870 and so on. So you can, in theory, craft a system exactly to your price point.

Perhaps more significantly, in our extensive testing, pairing a Radeon HD4870 with a HD4850 was just about as fast as a 'true' HD4870 CrossFire arrangement. If this state of affairs holds true as drivers mature then you could end up saving yourself $l50 on a dual card set-up.

And that's the rub. Whatever test scores we get today, there will be improvements tomorrow: because getting multi-GPU rendering right is a fine art. For which reason alone, we're confident in saying that most people will be better off with a single card.

It's quite common for two cards to actually be slower than one. Once the CPU cap is hit, the extra latency the second card introduces with its additional processing steps takes a toll. At higher resolutions, there's a stronger argument, but you're essentially talking about framerate improvements of less than 20 per cent in real terms, especially once a bit of image quality is turned on.

So which would we recommend? SLI or CrossFire? Of the new cards, CrossFire scales better in some games than SLI, and vice versa. Where combining GeForce GTX 260s is the better option, the performance gains overall seem to be bigger too - so from a technological point of view we'd probably go down this route. However, two GTX 260s are very noisy and $200 to $300 more expensive than buying two HD3870s, too, so from a practical standpoint that would be where our money would end up going.

Given the enormous number of shader units involved, two HD4850s seems like a bargain powerhouse on paper, but in truth it's not a lot quicker than a single HD4870, and all the arguments for a single chip over two sway us in favor of the latter.

In fact, across the board we'd say hold off on building an SLI or CrossFire rig for the time being. For one thing the forthcoming X2 and GX2 versions of the HD4870 and GTX 280 respectively will be better value for money, and for another, the performance gains just aren't worth it yet. A single GTX 280 will cost less than any dual-GPU set-up barring two HD3850s, and will still outperform them.

Give it time, though. There is one group of people who definitely should be looking to go SLI in the near future, and that's the many, many people who bought a GeForce 8800GT.
It's the marketing story the manufacturers aren't bragging about, because if word got out it could really hurt sales of the new cards. Thanks to driver maturity the gains from pairing two of these are enormous: put simply, in some cases two 8800GTs are cheaper and faster than a GTX 260, and faster than an HD4870. Plus, if you own one, a second card will only cost you the same as a single HD4850 right now - but be quick. It's likely to start vanishing from shop shelves soon.

It's likely that the current generation of cards will begin to scale as well as the 8800GT in the not too distant, but why take the gamble? For the first time in graphics history having the latest and greatest may not be the best - and we have SLI and CrossFire for all their flaws to thank for that.