In general - it depends. ;)
All CPU's and GPU's should have a maximum thermal limit specified on the manufacturers website. For example, this processor has a recommended maximum thermal limit of 75C. Although some GPU's don't specify, a generally acceptable maximum upper limit for GPU temperature is 90C (even though certain CPUs/GPUs can technically go higher, this is probably way too hot to sustain under load).
If your temperatures are really concerning you, then you should consider cleaning all of the dust buildup in the case, adding another fan (or repositioning the older fans), and either getting a new CPU heatsink, or re-seat the old one with new thermal paste (which dries out over time).
An absolutely essential tool is HWMonitor - it will provide you various temperatures of your computer from almost all sensors inside of it, even hard drives.
Also, remember - your fans should suck air OUT of the case. Try to get a cross-ventilation happening, with air moving through the case from one side to the other.
Back to the CPU's now. Pentium 4's have an upper thermal limit of 65-78C, with the lower limit obviously preferable. If you hit the limit, there's a few things you can do:
- underclock the processor
- reduce the core voltage (may have to underclock or lower the multiplier to stabalize it)
- get a new heatsink (see above)
- put more fans blowing air onto/moving air away from the CPU
Since you use Debian, you might want to try the GNOME Sensors Applet, although I'm sure there is a million other temperature monitors out there that you could use.
You are running Ubuntu so the normal suggestion of Speedfan is out, but you can look into two software packages, the first is called lm-sensors which provides with the temp sensors and fancontrol which gives you the ability to control your fan activation ranges. I am not a native Ubuntu user but if you Google those two you can find tutorials on how to install and setup the applications. Hopefully this helps.
Best Answer
It is neither a bad idea or a good idea. A PC is designed to be turned off, put to sleep, and to be left on. It being used at 100% load or 0% load makes no difference, it is designed to be used, any failures will not be linked to the type of work it does.
If electricity is being ran through a circuit, and the temperature of the corespondents are within their specification, then having no electricity and the temperature still being within the specifications makes no difference. Your PC will cool itself off faster, without being turned on, for obvious reasons. I have left my last 4 computers, spanning over 20 years, turned on 24/7/365.
If the system is cooled enough to keep the system stable for 12 hours a day then its stable enough to be ran 24/7/365.
There is nothing to worry about. The cooling system in a general sense does not care if the system is at 100% load or 50% load it will continue to cool the system no matter what.
You do understand, the hardware is not smart enough to do anything it was not designed to do, right?