SLI And Virtual Reality Applications
A word of warning is in order if you're using an Oculus Development Kit 2, or are planning to purchase the Oculus Rift once it is released (it's rumored to be coming in late 2015). Virtual reality headsets, by nature, rely on the lowest possible level of latency between head movements and display updates. Anything else can result in uncomfortable motion sickness-type reactions.
Nvidia recently introduced a "Virtual Reality pre-rendered frames" setting in its Control Panel (with a default setting of "1") to help reduce latency to its lowest level possible. Unfortunately, this setting does not apply to SLI; it only affects single GPUs. Because of the way SLI works, the CPU needs to pre-render at least two frames at any given point in time for a measurable performance benefit.
The Oculus DK2 supports refresh rates no higher than 75Hz. That means the minimum displayable frame time with v-sync is 13.3 milliseconds. Pre-rendering an extra frame from the CPU results in an additional latency of the same amount, which sounds small but is actually quite significant to VR.
Note: Without delving into a rather complex discussion of why v-sync is essentially necessary to VR, or debunking the common "v-sync is evil" attitude in the desktop gaming space, trust us on this one. You'll want v-sync on in your VR applications.
In situations where your game cannot be rendered at a constant 75 FPS, having SLI enabled will help. A frame pre-rendering delay is always desirable over the stuttering caused by having v-sync enabled and "missing" a frame refresh, thus showing the previous frame again.
Furthermore, if the latest Crescent Bay prototype from Oculus is any indication of where the consumer version is headed, maintaining a 90Hz (90 FPS) frame rate at 1280x1440 per eye with v-sync enabled may in fact require two cards in SLI. In that case, the minimum displayable frame time with v-sync is reduced to 11.1 milliseconds, which is certainly an improvement.
In short, SLI is neither required nor necessarily desirable for the current DK2 version of the Rift. But it might actually be helpful for an ideal experience once the final consumer version is introduced, particularly if you want to play with the eye candy turned up in current-generation games. If you're shooting for the absolute lowest possible latency, however, you'll want instead to drop your detail settings and go with a single GPU.