Two moderate cards in SLI will just about always top a single high end card. For example, two 650 Ti Boosts for $310 outperform both the 680 ($400) and and 7970 Ghz ($450) ..... Techpowerup didn't seem to have any of these driver problems and in-game crashes and neither have I.
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_650_Ti_Boost_SLI/23.html
After running the GeForce GTX 650 Ti Boost SLI through our test suite, I have to admit that I'm impressed. The duo delivered performance easily matching and often exceeding much more expensive single-card options such as the GeForce GTX 680 and Radeon HD 7970 GHz Edition, and they don't cost as much. SLI multi-GPU scaling works well with all of our titles except for F1 2012. Scaling by going from one to two GTX 650 Ti Boost cards is around 70%, even with F1 2012 taken into account. Unlike AMD, NVIDIA does a good job of maintaining its SLI profiles, so you should be able to play new games without a long wait for multi-GPU support. However, the risk that a game will not be supported still exists, and you might, at worst, end up with single-card performance. This is in my opinion, given the massive performance-per-dollar advantage, an acceptable tradeoff. I would definitely recommend a GTX 650 Ti Boost SLI setup to a friend looking to spend as little money as possible on a high-end gaming rig.
With a combined price of [$310], the graphics cards cost much less than the HD 7970 GHz Edition ($430) and the GTX 680 ($440) while still delivering comparable performance. Power draw and noise levels are slightly higher, but that's the price you'll have to pay to save over 100 bucks. This setup also makes upgrading your aging rig to play the latest and most demanding titles without breaking the bank an option..... Have [$310] and want high-end performance? Consider this!
Yes, when a new game comes out, ya likely gotta wait a week or 3 until a profile gets written and tweaked, but other than that, my experiences have been smooth. The twin GTX 560 Tis ($410) on Son No. 3's box smoke the single $500 GTX580 on Son No. 2's box by over 40%. Does the SLI box crash with driver issues ? ..... certainly ..... but no more often than my lappie (670MX) or the 580.
The instances where I have seen issues with SLI have been with overlcocked GPUs and cheap PSUs (read as nothing that doesn't get a 9.5 or better rating on jonnyguru). For whatever reason, a system that needs a 450 watter will often be fed by a 650 watter in single GPU build and yet when they go SLI/CF, the wattage is oft close to the bare minimum.
The ATX spec allows 5% voltage variation from the CPU and if that's what you're seeing, then I expect that one will experience crashed. As the game demands rise and fall, the PSU scrambles to supply proper and stable voltages. The VRM on the cards tries to manage this wildly varying feed and struggles doing so. With anything more than moderate overclocks, I look for 1% voltage variation or less ..... typically a 10.0 performance rated PSU on jonnyguru (Corsair AX series, HX series ... except for 1000 and 1050 watters, Seasonic X series, Antec Signature or CP series. Those with 9.5 ratings (XFX Core Editions, Corsair TX V2 series, Antec HCG / HCP series will do 2-3% and handle a bit more moderate OC's. Corsair CX series, Antec Neo, Basiq, etc., Thermaltake, Coolermaster, OCZ etc are going to have much more difficulty handling the more variable loads from SLI and will therefore be a bit more problematic. NOTE: I have also seen power conditioning on the house line eliminate or at least reduce such problems..... the more stable the feed to the PSU, the less it struggles and therefore the less everything down the line struggles.
That being said, a lot depends on budget as to whether ya go SLI from the getgo or get one now and one later. With $300 to spend, I think Id get the twin MSI 650 Ti Boosts (2GB) @ $155 each for a total of $310. At $400-ish,\Id get the Gibabyte Windforce GTX 770 and a PSU that could support a 2nd further on down the line.