AMD Athlons have a max temp around 97°C from what I've read ( I believe that is on die temp, not what BIOS reads... theres a 20°C (?) diff. between the on-die and BIOS temps, so you actual limit is probably around 80°C or so), so unless your temps are
really high, you don't have much to worry about. Now that said, having high temps and overclocking don't mix... you get system instability and crashing when your cpu temps are too high.
Also, Asus boards tend to read temps 5-10°C low, so the temps that you're reading now are lower than your actual cpu temps... so keep that in mind when you oc'ing
Most of us like to keep our temps under 55°C load just for practical oc'ing (that's for P4's, I don't know what the AMD guys consider practical ), and if you're crazy about temps like me, I don't like my cpu temps going over 45°C load... but that's a personal quirk
You can lower your temps by cleaning up your cables and cutting out the case fan grills... replace the rear exhaust grills with wire grills if you worry about chopping your fingers off
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Here's some pics of my case if you want some idea of what I'm talking about
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