As it stands now, most gamers are using 1080P displays in which case a rebranded 7850 (R9 270) or a GTX 750 are fine for average consumers. More are adopting 1440P and higher refresh displays up to 144hz and we are starting to see more overclocked 1440P screens hit some popularity. Still, the GTX 770+ and R9 290+ can handle most situations OK with a single 780ti being solid for everything up to 4K 60hz and being decent.
That was fun until what is coming over the next few months. Both SLI and Crossfire are iffy, under ideal circumstances and 2 780tis or 290xs are needed for full 4K 60+ FPS in new games with a bit of leeway for gsync and mantle/DX12. However, these 4K/120hz monitors will soon hit market in bulk and will be cheap. Even without any real competition for Nvidia, just the demand for cheap, much more powerful cards will force their hand.
For occulus rift, the requirements are higher because a 90+ FPS means no headache, nausea and loss of immersion. Two 780tis today are needed for the dev kit 2 to run nicely. Again, this will hit mass market soon (next year) and WILL sell well. Nvidia knows they better have cheap hardware that runs these products nicely or people like Zuckerberg will become card producing competitors.
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occulus rift 120hz oled displays reasons vid cards makers push limits