Depends on the 120mm radiator and the expected wattage of the GPU. Also a little bit on the block. If you are going to leave the GPU at stock it is only rated at somewhere between 185W and 220W, depending on the model. That would be total output for GPU, VRM, and memory, and even fan current. If this is a generic block that only covers the GPU it should be somewhat less for radiator to deal with.