Wow, nobody replied to you yet?
NB Core would be the one you want to increase, but it might not be necessary.
I have the same board and haven't tried to overclock the gpu portion yet, but I have seen reviews with other boards and they were able to overclock the gpu to around 900 pretty easy without a voltage bump. I would try 800 or 850 and see how that goes first before trying to change voltage.
Working with this board is frustrating to me. If I pick XMP for my memory, then changing the NB voltage causes the board to not boot. I don't know why. I have the same F6 bios as you.
I was trying to increase the NB voltage to make my 2400Mhz ram stable. I can't get it to work no matter what I do.