assuming your case ambient temp is around 30c, with top of the line watercooling your idle temps will drop to about 30c, and your load temps will drop to about 50c. it also depends where you put your radiator, if inside or outside the case. I am assuming you are going to put it inside, so your ambient temp is 30c. if you place it outside, and your room temp is 25c, then your cpu temps are obviously going to be lower.
what makes the difference with water cooling is your core temperatures. the delta between IHS and core temperature is drastically reduced with water cooling. if you idle around 30c, you can expect your 4 cores to be around 35c, and if you load at 50c you can expect them to be below 60c. with air cooling this doesn't happen. the delta is usually greater than 10c.
if your cpu is running at 65c while doing prime95 small FFT, it means that under normal load it probably runs around 50-55c. if that is the case, I strongly discourage watercooling, because its very expensive, and its not the easiest to setup. if that is not the case, then you may want to look into watercooling, at which point I pass the ball to aigomorla and I recommend you send him a PM.