Greater Graphics

Two Cards or One, That Is the Question

PCs featuring dual-card graphics capabilities like nVidia's established SLI (Scalable Link Interface) and ATI's still-evolving CrossFire let users run two graphics cards simultaneously, based on the assumption that two are always better than one. However, our tests reveal that this isn't always the case, particularly at the pricey end of the graphics card spectrum.

We tested both a mainstream and a high-end SLI graphics card combination using Alienware's Aurora 7500 SLI system. At press time ATI CrossFire-based systems were still unavailable. For the mainstream assessment, we tested cards with nVidia's GeForce 6600 GT that sell for about $165 each; for the power graphics comparison, we used GeForce 7800 GTX cards priced at about $550 each.

At the high end, tests at a resolution of 1024 by 768 actually favored the single-card 7800 GTX over the dual-card setup in some instances. For example, in our Battlefield 2 test with antialiasing turned on, the single-card version hit 78 fps and the dual-card one reached 76 fps. However, when we turned up the resolution to 1600 by 1200, the dual-card option took over, posting 69 fps versus 58 fps with antialiasing turned on. Half-Life 2 results were similar.

In our other tests, such as Quake 4, the dual-card setup generally outperformed the single card at the lower resolutions, but often by an imperceptible few frames per second. When we turned up the resolution, the dual-card option widened the lead.

Mainstream Cards Show SLI Advantage

The single-versus-dual showdown using the mainstream 6600 GT boards produced a clearer winner: Without exception the dual-card configuration bettered the single-card option at both resolutions. However, during testing we received a $200 card based on nVidia's new GeForce 6800 GS chip, so we threw it in the mix and found that for just $35 more than the price of a single 6600 GT (and $130 less than the cost of two), the 6800 GS proved a worthy challenger to the dual-card 6600 GT configuration.

In Battlefield 2 at 1024 by 768 (with antialiasing turned on), the single 6600 GT card posted 40 frames per second, the dual card-6600 GT setup reached 59 fps, and the single 6800 GS card notched 60 fps. In the Quake 4 test the dual-card 6600 GT setup won out at the same resolution, reaching 111 fps compared to 67 fps for the single 6600 GT card and 84 fps for the single 6800 GS card.

So when are two cards really better than one? At the high end it makes sense to pay for two 7800 GTX cards only if you need the very best level of performance and you plan to run your monitor at 1600 by 1200 (or higher). If you're in the mainstream camp, the recommendations are trickier. If you already own a 6600 GT, buying a second one makes sense. If you're starting from scratch, a single 6800 GS is the smarter buy.

Tom Mainelli