It's A Great Time To SLI
Nvidia's SLI is in great health as of early 2015. AAA games don't always support SLI properly on launch day, but they either get patched quickly or run well with minor workarounds. Once a proper profile is in place for a game, two-way SLI yields real-world scaling of 75 to 85% at 3840x2160 on our GeForce GTX 980s. Below 4K, you'll probably be CPU-bound in most real-world metrics.
Two 980s in SLI complemented by a decent CPU, can, in many cases, push above 150 FPS at 1440p, effectively powering even the most advanced 144Hz gaming displays commercially available. Overclocking, which isn't as straightforward with SLI compared to a single GPU, can give you an additional 15-20% boost if you want to go that route. The Maxwell architecture's high efficiency paved the way for quieter cooling solutions, making SLI even more attractive this generation.
So should you consider going multi-GPU with SLI?
If you're gaming at a resolution/refresh of 1080p/60Hz or lower, you don't need it. One GeForce GTX 980 (or 970) maxes out pretty much everything you throw at it these days. But if you're eying 1080p at 120Hz or more, 1440p at 60Hz+, 4K or gaming across multiple displays, two or three graphics processors will help achieve the performance levels you want. Just know that you'll lose certain features along the way, and will probably run into technical challenges. Further, micro-stuttering may still be an issue for you at the highest resolutions.
SLI is also a great option if you purchase a single GeForce GTX 960, 970 or 980 card now and plan to upgrade at some point before the next-gen architecture surfaces. A second identical Maxwell-based card will almost certainly yield your best bang-for-the-buck upgrade. Just keep in mind it really does need to be identical though, and the inventories of some GPUs don't always last long. Reference-class cards are probably a safer choice in this situation.
We tested for micro-stuttering and found that there really is none to speak of, at least from our one configuration, at 1440p. But there are still issues with micro-stuttering at 4K.
Early adopters already considering their upgrade path for VR might want to hold off. SLI is not a great choice today for the Oculus DK2, but it might be a viable option for the final Oculus Rift when it surfaces as a retail kit.
Finally, if a dual-GM104 card is in the works, and if Nvidia decides to price it reasonably, unlike what it did for GeForce GTX Titan Z, we'd definitely be interested.
In part two of this series, we'll look at the red team's rival multi-GPU technology called CrossFire (we've already lined up a couple of AMD's next-gen cards to make this one happen for you). We'll also compare how the two companies' flagship cards performance in a two versus two royal rumble.
So stay tuned!