I used to run my Sager 5680 (2.8ghz P$ w/ 800mhz bus, 512 mb Ram, MR9600 128mb ram) at full load all the time. Play video games during the day, encode at night. Didn't have any problems except it would get hot. No artifacts or crashing problems.
However, the load your laptop can take is directly related to who built it and its design. Some notebooks just start lowering clock speeds when they get blazing hot. My Sager never did, but then again, nothing ever burned out either so its cooling system was adequate.