As has been said, Pascal is coming soon, and is expected to have greater performance increases than the Kepler to Maxwell transition (Pascal should be released to public around the end of Q2). However, I wouldn't dismiss multi GPU setups out of hand, though perhaps it makes more sense as a later upgrade.
Why multiple GPUs are better with Pascal:
- Pascal Introduces NVlink, which can be used to connect GPUs even when
the CPU does not support it as in the image below (image source)
- Rendering generally scales well with multiple GPUs. SLI is not required.
(sources: iray benchmarks, VRED Nvidia article, Vray RT Nvidia article)
- With the introduction of DirectX 12, Split Frame Rendering is making a comeback. Multiple GPU solutions with good game support can have comparable frametime latencies and also allows the graphics cards to not duplicate all the resources across the two memory pools.
Why you might opt for a single GPU anyway:
- Upgradability. You could always drop in a second later. Third GPU scaling is way worse, though that might change with NVlink, a single GPU is still the better option in that respect.
- Support. It's annoying when sometimes SLI doesn't work for a game, and you're stuck with poor performance. You'd also need a slightly bigger PSU and case, because lower end cards are higher clocked, and two obviously take more space.
What you should also consider:
AMD's Polaris is also releasing around the same time, and is supposed to have competitive performance per watt. Of course, you might run into problems with vendor specific software, such as those using CUDA, but it goes both ways (well, slightly more in favor of Nvidia, owing to their market share), and AMD hardware generally have better OpenCL performance. If you're not worried about power consumption and you want your computer now, 2 R9 390s would be decently better performing than 2 GTX 970s at high resolutions.
