I have a Gateway 600S with the Mobility9000 on it and I can overclock it from 250/200 up to 275/230 with no artifacts of any kind in the middle of summer with no air-conditioning.
I tested it out by running loops of different demos and benchmarks and it works perfectly
It all depends on what type of cooling it has and what else is going on.
I also modded the cooling system on the P4 a little bit as well though so that might have a little to do with it since the CPU isnt making the temps sky high