can't really say much as far as heat goes, as alot of the components inside a pc are not happy with it, but generally the silicon parts in a pc (north and south bridges, gpu, and cpu) are capable of running up to around 75c without breaking down... though a "safer" number would be a max of 55c loaded, or 131F.
Now cold on the other hand, look at all the overclockers in the desktop area of computers, with their north, cpu, and gpu under peltiers with watercooling, and even short runs of ln2.... only things that should get finicy with extreme cold is the hd (fluid bearings) and if a few of the misc small chips on the mobo get very cold.
These are all running conditions. If the unit is off, heat nor cold is a big issue. as it would need to get hot or cold enough to break down the parts itself, and 140F won't do that without the computer itself adding to the heat.