You need to find out what the max voltage is for your card.
1. open Afterburner
2. set the core slider to any value (multiple of 13) so 130, 143.. etc. apply
3. put your mouse in the graph window, hit cntrl-F
4. select the 1025mV, 1050mV or where the graphs slope and straight lines meet point and hit cntrl-L
6. save so you can set your max v any time you like.