What ever is needed to run as fast as your want it, although you must have enough cooling.
I'd have my 2500@ 2.15 V, havent really tried a lot with my xp-m 2500 now.
1.7 as a rule of thumb is where people generally stop due to SNDS, but many people regularly run them over that voltage... it depends how ballsy you feel.
i won't go over 1.675 on a NW but my old P4 1.4 i run it at 1.9v all the time. 1.675 is my max for the NW and over 1.7v is goin into SNDS territory which is a bit scary for a student like me who does have much money to burn. and going over 1.7 volts isn't gonna provide alot more stability so i don't see the point