From what I've read here, everyone mostly thinks under 60c is best for the Athlons. However, I don't know if that is core or die temp... (anyone?)
There's 2 different CPU temps on many mobos. The core temp and the die temp. AFAIK the monitors that run in windows usually read the die temp, which is several (10%?) deg cooler than the actual core. (Core is on the chip itself, die is just below the chip in/on the socket)
I have a barton 2600+ oc'd to about 3200+ speeds (195x11.5) and even when it gets warm in the room (probably over 25C) and under load, the die temp stays under 50, with the core about 55. So, I'm guessing I'm pretty safe.