Stability is your only limiter unless you plan on physically moding the mobo to get more than 1.85V. With that hsf, you should be able to happily run your duron 600 to 950 /1.8-1.85 volts and if you have a good chip, It just might make 1ghz. Temps will raise but still be well within limits.
Like I said go by your stability at any given voltage to be your guide. Personally if your current OC is fine with you and your system is stable with stock voltage then all the better. Will run cooler and last longer.
BTW- I test with Prime95 for stability for my CPU and mobo.