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.

Right processor

How to pick the right processor?
This is a hard question. You have tradeoffs between price,performance, compatibility, upgradebility, and power consumption. As a desktop unit owner, you probably have less concerns about power, but as a laptop owner, this is very important.
The frequency of the CPU defines how fast its internal clock runs. This defines how fast instructions are executed. In many ways, this is meaningless, because a RISC machine (MIPS) running at 100MHz may in reality be slower than a 50Mhz i486 because a RISC system must execute more instructions to perform the same function (in some cases). Even when comparing processors in the same family, this info can be misleading. For example, an Intel486-25 is faster than an AMD386-40, since the 486 has microarchitectural advancements over the 386. The same can be said for the Pentium, where a 66Mhz Pentium is twice as fast as a 66MHz 486.
For compatibility, keep in mind that the Intel parts are the basis for all of these processors. Therefore you always run the risk that an imitator's part may not be compatible. AMD [486] chips are compatible because they are copied. For some of you, these factors may be important.
As far as upgradability goes, this depends on both your motherboard and the processor. If you purchase a 486DX, then you can upgrade to a DX2 and double your internal clock simply by buying an overdrive chip if your motherboard has the ZIF socket. If it doesn't then you canreplace the CPU with a DX2. Many new 486 motherboards containoverdrive sockets for the Pentium chip that is pin compatible.