what would be a max voltage i should go to?
As you overclock and overvolt, you have two things happening. One, errors, two, excessive power.
Errors of course will give black screens, sparkles, etc. Overpower will cause the GPU to slow itself down.
If you way overvolt, you may exceed the power of the board, even with a stock clock rate!
Find the maximum clock rate at stock voltage.
Bump the voltage just a bit, 25mV or even less.
Find the maximum clock rate again.
Repeat until you get the fastest clock rate, you will run into either errors or throttling down.