som small tips

Should I turn my computer/monitor off?
Turning a device on/off causes thermal stress. Leaving it on causes wear and tear (even on non moving parts). The only thing you shouldn't do is quickly power cycle the computer. If you turn it on/off, leave it that way for at least a few seconds. Other than that, it's up to you.
What should I upgrade my omputer first?
This is highly debatable. In general, if you buy from a good, self-respecting company, any additional cooling requirements would have been taken care of before your system was delivered to you.
I'm responsible for PC maintenance at my site, and I don't worry about additional cooling in any of the pre-packaged systems we receive. All rebuilt and/or upgraded 486-33 (or higher) systems do, however, receive additional cooling because older cases may not provide adequate ventilation for today's technology. Additional cooling on the infamously-hot Pentium (586) is always added.
So, do YOU need a CPU fan in YOUR system? Probably not for "ready to run out of the box systems." If you are _REALLY_ worried that your system is suffering from too much heat, consult with a reputable service center. They will not only answer your questions, but they can also install any additional cooling systems that may be needed. A good rule of thumb, though - "don't try to fix what isn't broken."In other words, if your system is working, don't look for trouble.