The chaps over at OCC have published a new review on a Crossfire vs SLI Performance Comparison. Here's a bit:

So what did we learn from this little exercise? I learned that SLI is the more mature multi-GPU solution currently. In each of the three classes, the Nvidia technology and their video cards lost no more than four times out of 40 tests, with all things being equal - this being in the quad GPU class. Each of the other classes delivered a 37 to 3 margin of victory for team Green. Each set of cards was run at the default factory settings in the control panel to show what Joe Average will get out of a plug and play system. Could performance be improved upon by manipulating the settings in the control panel? Sure it could - on both counts, but this was about running what the system defaults to. The only deviation from this is in 3DMark Vantage, as the Nvidia cards share a distinct advantage in the PhsyX tests. The reasoning is that Futuremark does not allow the scores when the GPU does the PhysX calculations, because ATI does not have an equivalent technology to compete right now. Right now, SLI is the way to go for multi-GPU performance in the games tested in this comparison. The downside to this is that it will cost you a little bit more to get this performance, as ATI seems to have the pricing game down pat at the performance levels I looked at here. With just raw cost as a factor the GTX 295 quad GPU setup will set you back just over $1100, while the HD 4870x2 combo will only (insert sarcasm here) set you back $800-$1000 depending on the cards you buy. The Sapphire cards will only set you back 400 bucks each, instead of the more popular pricing of 500 bucks. In the single GPU card class, the GTX 285 combo will cost you almost $700 - at a cozy $680 - while the HD 4890 combo only goes for $530, with prices scaling downward based on the video card's capabilities. That being said, the best value per frame per second delivered is the GTX 260 SLI combination at $3.44 per FPS at 1920x1200, and $4.59 per frame at 2560x1600. In most of the games tested, the GTX 260 SLI combo did not deliver the best performance, but it was able to hold its own. Raw performance goes to the Quad SLI GTX 295 combo.

For most of us, at some point the power bill gets to be a concern - especially when you run a distributed computing project on both your CPU and GPUs. The power company wants their piece of the pie just like everyone else. While our test systems are not the most power hungry, it pulls a decent amount of power under load. The loser in the power consumption testing has to be the HD 4870x2 CrossFireX setup. With our systems and these two cards under load, the system pulled a total of 936 watts from the outlet. As a comparison, the Quad SLI setup used 757 watts from the mains. This scenario continued when the dual GPU cards were pulled out of multi-GPU mode, with the 4870x2 pulling 656 watts, and the GTX 295 pulling 471 watts under load. In the third class, the GTX 285 SLI combo did pull more current than the HD 4890 combo, with the GTX 260 setup falling between the HD 4870 and HD 4850 CrossFire setups.

When you look at the scaling in performance you get from going to a quad GPU setup, it is not going to be anywhere near 100% in most cases. There were a few instances where the scaling on the HD 4850x2 and HD 4870x2 scaled closed to 100% - which was a real surprise. This was the exception, and not the rule. In turn, there were games where there was no scaling whatsoever with cards in CrossFireX, while the SLI combo scaled well. That's something that could be a driver fix, to enable better CrossFire support in games. To use all this video horsepower, you will need a monitor that runs no less than a 1920x1200 resolution, as anything else means you are just throwing money away. The ideal solution is to go with that 30+ inch monitor you have been lusting over to take advantage of the firepower. Currently, Nvidia and its SLI technology is the performance winner here, on this system, with the drivers and video cards tested. The numbers are the numbers, and this is what they show. ATI has great price points, but they just can't deliver overpowering performance for that price. On the other hand, Nvidia delivers the performance but at a steeper price. The old adage "you have to pay to play" comes to mind here. By paying, you have a GPU that is capable of doing much more than just create pretty pictures for us to look at on that magic screen. With Nvidia's CUDA technology, you have a wide array of applications ready to take advantage of the parallel computing capabilities of its architecture, such as vReveal from Motion DSP which allows you to clean up poorly shot video, Badaboom for converting video to most popular mobile formats, Folding@Home where performance is just amazing, with more coming each week, it seems. You have GeForce 3D Vision to immerse you in the game. GPU Acceleration in Photoshop! What more do you need? Drivers? Why yes indeed, Johnny. Nvidia seems to be more committed to delivering drivers almost as fast as I change my underwear (yes, it's a daily occurrence), while ATI is still stuck on a once a month schedule, and you need to hope and pray they work. One thing that could overcome the performance problems is having user adjustable CrossFire profiles so the performance is there. When there is no scaling or negative scaling, a profile for that game may be all that's needed to excel, but currently that's not the case. Price is a point of difference, but there is more to it than a GPU that costs less but offers less. You don't pay Hyundai dollars and expect Corvette performance. Sometimes you get a surprise, but now its ATI's turn to swing for the fences. The potential is there on the CrossFire side of the fence.