to monitor you CPU and Mainboard temperatures - installed on most modern distros:
And use an external thermometer to measure the room temperature. hddtemp
can get the IDE drive temperatures.
If you want to monitor your readings over time look at rrdtool
- you can set yourself up with time series data sets and graph them - scripting skills will be required. But pen and paper notes are probably good enough for tuning.
Do some stress tests, such as kernel builds, and see how the temperatures fluctuate allowing for the room temperature. Google should help in finding what good temperatures to expect for a given CPU - beware that some overcloaker web pages tend to want things really cool - so you can aim for a few degrees above where they aim.
I have an Athlon XP 1700+ - people have reported that Athlons can start to get unstable somewhere above 55C - but some chips are quite stable above this - luck of the draw - mine seems 100% stable at 58C, a friends Athlon survived 100C when his CPU fan failed. My two hard drives seem to run in the range 32-35C. My CPU idles at about 46-48C. My mainboard idles at about 36-38C.
AMD recommends a vent fan below the power supply moving air out of the case - and also recommends against inlet fans at the bottom of the case - they claim this can actually reduce cooling to the CPU - but they're probably ONLY worried about the CPU.
I wanted a quiet machine - so high reving fans are not on my shopping list. I found that adding an extra low rev cooling fan below the power supply venting out did improve all the temperatures. I eventually switched to a low noise (low rev) CPU cooler. When a really hot summer kicked in, I removed the entire plastic front panel and took base measurements with front to back airflow at the maximum. I then mod'ed the front panel by enlarging some downward pointing inlets (nobody can see them - so it looks OK). I then replaced one 5.25" bay cover with a 5.25" cover designed to accommodate a 3.25" peripheral, but instead of a peripheral I added a grill made from an old PC-speaker grill. With the extra air inlets the system now runs within 1C of the base measurements. If I ever buy another mid-tower I'll look for large speaker-like front grills with dust filters.