It should run in the low 40's with average cooling. Thermal tape on the HSF will run hotter than t-grease. Most people seem to be able to keep the beastie under 50 when oc'd and under stress with a good HSF even at v1.85.
Supposedly the limits are alot higher, but I haven't seen a duron/athlon system run "stable" at over 50c idle.
When I first got my system (asus A7V, duron 650) it ran at 55C with the default cooler. I never had any stability problems. Now i have a chrome orb and it runs at 800 at 52C. Still never any stablility problems. I have heard of some people who run their processors at 65 and don't have any problems, but I wouldn't recomend it.
Duron 600@950 on an A7V.
Runs between 41C idle and 48C after hours of gaming.
4 case fans (front air in, side air in, 2 back air out)
2 fans on the power supply (one on the bottom right over the processor, nice!)
1 video card fan
1 cpu heatsink/fan (GlobalWin FOP32)
Guy at work, using fan that came in the AMD processor box and he runs a 750 TBird at about 66C idle, who knows what it gets to after doing heavy work. But he never has problems with stability.