Which chip are u using? Varies across the different generations. Also depends on how much u intend to OC, if u want to hit the absolute limit (before LN2), it's advisable to get a 240 40mm. However, if u just want to OC a 0.2 or so, I recon a 120 40mm would be enough. Aside from the radiator, the fan used also important as it is the main source of heat dissipation (maybe can get a 2150 or something RPM and set your own fan curve)