default voltage will be 1.75,  but anything up to 1.85 is pretty safe
as for how high you can overclock it....  there is no way to tell except just by overclocking and seeing how high it can go.   if its a pretty good sample you should be able to get 1.4ghz or higher
just make sure you have a good heatsink/fan and pretty good case ventilation
if you plan to just use stock heatsink/fan you may want to try a smaller overclock,  maybe something in the 1.2 or 1.266 range (that would be the speed with 133mhz fsb and 9x and 9.5x multiplier)