As far as Athlon temps, I actually hit 80c and never had a lock-up when I first built my PC. I kept increasing my FSB while using the retail HSF and one case fan! Like these guys say Athlon temps mean nada until it locks up. Also, different chips (two XP 1900's for example) might run at different temps on the same system, and might lock-up at different temps. Lots of variables. HOWEVER...
If you would like to lower temps just because you're bored and still are worried (plus it doesn't help when people post their XP's running at 40c with air cooling, yeah right!) take it from someone who's been there. With MBM 5.0 (thanks to PooBaa) I idled at 53c, and usually went up in the 60's when doing something intenstive. I didn't have my cables wrapped well, didn't have good caseflow, etc.
NOW...I use a Volcano 7+ HSF on medium setting, AC3, taped all cables and case holes, one intake fan (low, front) and two exhaust (all 80mm fans). What REALLY cooled me down is the blowhole cut I made to the top of the case (roof). My exhaust on the back of my case just couldn't keep up, and the blowhole exhaust gets rid of any trapped hot air up top (since heat rises). I used a Craftsman Rotary tool to cut (and it did take me 20 minutes and 5 disks).
Now I run a 150 FSB with a 12.5 multiplier on an XP 2000 and never go above 56c (but I also have an air conditioner..., so I have low ambients to start with). Now I'm much happier, feel better, and am able to get decent overclocks. Good Luck.
<font color=purple>Pentiums are for wusses who are afraid of instability</font color=purple>