The GeForce 8800 in SLI

For when too much is not enough
by Scott Wasson — 1:46 AM on January 23, 2007

WHEN WE REVIEWED the GeForce 8800, I said we'd test the GPU in an SLI configuration "as soon as we can." I will admit that I've dabbled in CPUs a little too much, and our look at GeForce 8800 SLI has been delayed. However, I also wondered in that same review: "who needs two of these things right now?" That's a pretty good question given the GeForce 8800 GTX's astounding pixel-slinging performance, and something of a de-motivator for one considering looking into GeForce 8800 SLI.

But now I have seen the light. It's wider than it is tall, modulated by a series of filters, and about 30" from corner to corner. I'm talking, of course, about Dell's 3007WFP LCD. We need not invent a reason for GeForce 8800 SLI since display makers have already invented a darn fine one. A four-megapixel monster like this one cries out for the fastest possible graphics subsystem to drive it, and the GeForce 8800 in SLI seems like a perfect match. We've had a bundle of fun testing the two together and exploring the world of uber-high-res widescreen gaming.

We've also dug a little deeper into GeForce 8800 antialiasing, to see how it compares to single- and multi-GPU antialiasing modes. Even the vaunted quad SLI makes an appearance to take on dual GeForce 8800 GTXs for extreme bragging rights supremacy. The power meter in Damage Labs has been spinning like a Hillary Clinton campaign staffer in the wake of the Obama announcement. Read on to see how we put all of that power to use.

G80 SLI: Powerful, yet unrefined
The GeForce 8800 is still a very new product, so running pair of them in SLI is a funny mix of extreme, err, extremeness and as-yet-untapped potential. First, let's talk about the extremeness. We already know that a single GeForce 8800 GTX graphics card performs more or less on par with two of its fastest competitor, the Radeon X1950 XTX, running in tandem. With 680 million transistors, the G80 graphics processor has a formidable appetite for power and cooling. In GTX form, the GeForce 8800 has 128 stream processors running at 1.35GHz, a 384-bit path to its 768MB of memory, is 10.5" long, and has two six-pin PCIe power connectors on a single card, like so:

The GeForce 8800 GTX's twin power plugs

Running two of these cards together in a single system will require a grand total of four PCIe auxiliary power connectors, more than even most high-end power supplies can handle. We were able to get our test systems going in GTX SLI using our OCZ GameXStream 700W PSUs and a pair of four-pin Molex to PCIe adapters, but doing so ate up a connection on every Molex-equipped power lead on the PSU. We even had to share one lead with our DVD drive, a less-than-optimal solution—and this was on a test system with only one hard drive and no extra accessories. Those who are serious about building a system with dual GeForce 8800 GTX cards would do better to go with something like this one-kilowatt beast from BFG Tech.

Four PCIe connectors sprout from BFG's 1kW power supply

The BFG Tech 1kW PSU comes with four six-pin PCIe leads out of the box, along with enough rated capacity to power five or six 100W light bulbs in addition to your PC. Unfortunately, like most 1kW PSUs, this BFG Tech one doesn't have a larger, quieter 120mm cooling fan.

The GTX's little brother, the GeForce 8800 GTS, doesn't require such extreme measures, since it comes with only one PCIe plug per card. These cards are no longer than a Radeon X1950 XTX or a GeForce 7900 GTX, either. Still, with 96 stream processors clocked at 1.2GHz and 640MB of memory behind a 320-bit interface, the GTS isn't exactly warm milk—more like Diet Mountain Dew: toned down, but still extreme.

A pair of BFG Tech GeForce 8800 GTS cards

Now, let's talk about that untapped potential. Current GeForce 8800 drivers support dual-card SLI configurations, but no more than that. Yet every GeForce 8800 card comes with a pair of SLI connectors on board.

Dual connectors promise big things in the future

A dual-card GeForce 8800 SLI rig will only use one connector per card. In theory, the additional connectors could be used in offset fashion to create daisy-chained configurations of three, four, or more cards, once proper driver support is available. The fact that the G80 uses an external TMDS and RAMDAC chip to drive displays even suggests the possibility of cards a la the GeForce 7950 GX2 with dual GPUs and one display chip or even of "headless" GPU-only cards expressly intended for use with SLI. (Of course, daisy-chained cards would in all likelihood have to be equipped with single-slot coolers in order to fit into any standard-sized motherboard.) I expect the cards and drivers will materialize over time, but they're probably not top priorities for the green team at present. They are, after all, winning the performance sweeps quite handily, and their G80 drivers remain a work in progress.

A more glaring omission is something I simply expected to see with GeForce 8800 SLI out of the box: SLI antialiasing. Both Nvidia's SLI and ATI's CrossFire can use multiple GPUs to achieve higher degrees of edge antialiasing than is possible with a single GPU. These antialiasing modes function as load-balancing methods that sacrifice raw performance for improved image quality. On the GeForce 7 series, SLI AA can deliver up to 16 samples with two GPUs and up to 32 samples in quad SLI. Similarly, CrossFire's dual-GPU Super AA modes reach up to 12 samples. Surely, I thought, with G80's nifty 16X coverage-sampled antialiasing, we'll see SLI CSAA modes up to 32X in dual-GPU configurations. Turns out that's not the case, at least not yet. Nvidia's drivers haven't yet enabled any SLI AA modes on the GeForce 8800. Support for SLI AA on the 8800 is in the works, but it's not here yet, and we don't have any ETA for it at present.

Fortunately, that's not a major problem, given the G80 GPU's excellent native AA support. We'll compare SLI AA and SuperAA to GeForce 8800 antialiasing in the following pages, and you'll see what I mean.