should i upgrade from strix 970 to r9 390 because of vram?

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It's more then enough. Here's a good article you should read when you have the time.

http://www.tweaktown.com/tweakipedia/89/much-vram-need-1080p-1440p-4k/index.html

"This test has shown that 8GB of VRAM is pretty useless right now, there's just no need. Sure, NVIDIA has a video card with 12GB of VRAM in the Titan X, and 6GB of VRAM on its new GTX 980 Ti, but most of the time it's not needed, even at 4K. If you start enabling AA, which we're going to be doing soon, then the VRAM consumption is going to skyrocket - which is something that will be interesting to see in our future article. For now, we've...
*rolls eyes* the 970 will be fine to play most any game for the next few years. A GTX 780 was recommended for Unity, and a 680 was the minimum needed. At worst I would say a 780 would be minimum and a 960 would be recommended. Ubisoft has also said they are aiming to have this game run 1080p at 30fps, so again, you shouldn't have any issues with your 970 except if they make a bad driver for it.
 



It's more then enough. Here's a good article you should read when you have the time.

http://www.tweaktown.com/tweakipedia/89/much-vram-need-1080p-1440p-4k/index.html

"This test has shown that 8GB of VRAM is pretty useless right now, there's just no need. Sure, NVIDIA has a video card with 12GB of VRAM in the Titan X, and 6GB of VRAM on its new GTX 980 Ti, but most of the time it's not needed, even at 4K. If you start enabling AA, which we're going to be doing soon, then the VRAM consumption is going to skyrocket - which is something that will be interesting to see in our future article. For now, we've shown you that even the latest games don't push that far over 4GB of VRAM, so you'll feel safe buying yourself a new card with 4-6GB of VRAM."

Also worth noting is that AA isn't really noticeable at 4K. I've tested this with BF4 on a 50" Samsung 4K TV using none to max AA and there wasn't much of a difference, the only change was that it was too much of a load on the GPU's and caused lag/stuttering.
 
Solution


So answer is 390 ! Article says 4-6 GB is safe. 970 has only 3.5 GB.