Topaz says RX 470 8GB or better is optimal. Is the RX 580 with 4GB better?
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The more vRAM you have the better, the 470 & 580 have approximately the same performance but the 580 is newer.
If your building a computer they have different connectors.
I have the RX580 card with 4GB of memory and it works very well. I normally enlarge up to about 6000 pixels but have tested a picture enlarged from 3264px to 23000px on the long side and it took 1 min and 47 sec. More memory won’t make it faster.
vRAM is dedicated to the GPU and will make a difference, but there is a physical limit for each GPU.
I realize that but if you are not using 4GB (which I have) then I don’t think 8GB will do anything more. I also noticed, when using the AMD Radeon software that shows the speed of the GPU and memory, that while the GPU goes to full speed when using Gigapixel, the memory speed stays near the lowest setting of 300 MHz. 1850MHz is full speed except I think there is a X4 multiplier somewhere.