I found this on AMD's site. You'll need Acrobat Reader to view it. On page 33 (Section 5 in the Table of Contents) there is a chart showing the maximum die temperatures for Athlons from 900MHz to 1.4GHz.

For those of us with XP processors, There is this. Go to page 35 (Section 6) for the chart.

Basically, all XPs and Athlons less than 1133MHz have a maximum core temperature of 90C. Athlons from 1133MHz to 1400MHz have a maximum temperature of 95C. These temps are measured from the die itself, so the temp you see in something like Motherboard Monitor is probably not going to be accurate. I would suggest staying below a reported temperature of 60C, with under 50C being my personal preference. Of course, lower will be better.