Let me explain to you how graphs work. See the one bellow my post? Also to be fair there's a graph from Tech Report which shows the 980 Ti beating the crap out of Fury X, but the Asus Strix GTX 980 Ti that they used for testing is Aftermarket factory overclocked that's 9% faster than reference clock.
So it's safe to say that the 980 Ti and Fury X are trading blows, but not like it matters cause who the hell buys a $650 graphics card anyway? Crazy people, that's who. And the Asus Strix GTX 980 is $870 at Amazon which means those buying it are crazier. Also just look at how AMD's cards are dominating the low and mid range.
Same reason why some 750 Ti's are sold with 4GB of VRAM, cause there's a lot of idiots out there that think memory is that important. Truth is no game today needs more than 4GB of VRAM. Most games are fine with 2GB, hell most are fine with 1GB. The only time you go beyond 4GB of VRAM is 4k+FXAA in Shadow of Mordor and 4k with AA is pointless. A 970 at that point should perform Seppuku.
The 390's with 8GB is AMD's way of saying they don't like the price trend of the 290's so they threw a useless 4GB more memory to keep the price high. Personally I would buy a 290 based on reference design and flash the BIOS to a 290X and crank up the overclocks. Better deal than a 390. If I want to I can flash a 390X bios on a 290, but it doesn't make a difference. A 290 can be had for $250, and that's not even looking on Ebay.
People use GPUs for things other than videogames. Using 3D apps like Blender, the more vram you have the more complex the scene you can render using Cycle.
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