Personally it depends on what kind of temps you are getting but the max I will probably put into my amd 2400+ will be 1.8 aslong as the temps are still good.
probabily 2.1v is the absolute highest at any point. i wouldnt run it at more than 2 for any length of time. i wouldnt run it at about 1.9 if i couldnt afford to immediatly buy a new one if it died.
that being said, AMD's are a LOT more voltage tolerant than P4's, so it wont be sudden death if you go too high
Been running mine at 2.0 V for the better part of a year (2.3 is highest I've used). Unless you have hardcore cooling, heat will limit your overclock more than any voltage maximum.
I don't go above 1.85, mostly because I find that with air cooling higher voltage than means too much heat for the CPU, and it becomes counter productive for overclocking.
Usually I don't even find 1.85 is better than 1.8, but it doesn't stop me from trying it .