GTX980Ti is roughly 150% performance and price of GTX980, so 2-SLI 980Ti is roughly the same performance and price as 3-SLI 980. Except that the 2-SLI would be better because it makes better use of PCIe 3.0 lanes, puts a lot less load on your PSU and mobo, would be easier to cool, and might be easier to fit into your mobo and chassis.
So I would buy one 980Ti now instead of a 980. And get a second one later, if needed.
But we've been anticipating NV Pascal cards and AMD R9-3xx cards for a while, lots of new cards with new GPU chipsets and exciting memory are expected to rollout Q2/2015 to Q1/2016. If you're looking to buy the best then it's probably worth waiting so you don't get stuck with a lesser card and a bit of buyer's regret for the next couple years. And faster factory overclocks are always evolving, you'll never catch up unless you replace your GPU card(s) every few months or deliberately buy an older GPU model - a Matrix 980Ti may or may not ever exist, but nonreference 980Ti and TitanX cards clocked better than the fastest available today are sure to appear. Don't worry about it too much, consider that the very fastest last-generation GTX980 cards (the 1304/1418MHz EVGA Kingpin and Galax HOF) are overclocked only 15% faster over reference. While many people successfully overclock reference GTX980 cards to similar or faster speeds.
If you just need a "decent" high-end card now and can wait to get an "ultimate" card a season or two down the road, then just buy yourself a reference 980 - the
Asus GTX980-4GD5 and
PNY VCGGTX980-4XPB-CG are both consistently strong overclockers, largely because they're 165W cards shipped with reference NVTTM coolers originally designed for 250W cards. Or, unless you specifically need G-Sync or some other NVidia technology, CrossFire a pair of R9-280X/285 cards. Sure, these cards are all outperformed by today's newest ultra-enthusiast cards (which will in turn be outperformed by next month's newest ultra-enthusiast cards), but they're still mighty beasts and they will retain most of their resale value over the next year.
But what are your system specs, what games do you plan to play, and at what display resolutions? You won't really need a 2-SLI GTX980 setup unless you'll play heavy games at 4K or across multiple displays, and you won't really need anything better than a GTX960 or R9-270 if you just play stuff like WoW.
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