i think aroud 1.85-1.9v would be the absolute limit
personally, my limit is around 1.8 actual if it yields that much more mhz
i can do 2200mhz now at 1.65v (1.69actual thx to asus lol)
if i can do 2500mhz at 1.8v, then i may consider it
but if its only like 2300 at 1.8v or higher, then screw it