"What are good temperatures for a computer?"
ANSWER: It depends on which processor you're using. Find out the manufacturer, model and speed of your processor, and pick through the support material on the manufacturer web-site (either Intel or AMD unless you run a MAC or a Sun Microsystems). Find the "Thermal Limit" of the processor in degrees Celsius.
At Idle, your CPU temperature should probably be well below 40C or 104F degrees. (The cooler, the better, the closer to room temperature, the better.) The upper limit at "load" (running PRIME95 "Torture Test -- Large FFT") should probably still be a minimum of 20C below the thermal limit, but the lower the better.
To give an idea of this, my Northwood C 3.0 Ghz processor idles at around 30C at a certain room temperature, and maxes out at about 44C under "torture-test" load. Bump up the room temperature, and these temperatures rise by an equal amount.
A Prescott (Intel) processor should run several degrees hotter than a Northwood, and regardless of the socket-design. AMD processors have traditionally been slightly cooler, because they (traditionally) have consumed less power and have less heat-leakage, but that seems to be changing with the newer models.
Also -- the Idle and Load temperatures are not as important by themselves (except insofar as the Load temperature being "high") as the difference between Idle and Load. A smaller change in temperature means less stress on the processor, so this difference between Idle and Load should be reduced as much as possible.
An Intel Northwood "C" processor, with decent cooling, should perform at "stock" speed settings below 50C under "PRIME95."
AMD processors are supposed to have less "heat leakage" (different from Thermal Limit -- the amount of heat given off at "load" in Watts) -- but AMD's newer processors are starting to close the gap on greater heat and more stringent cooling requirements in comparison to Intel. The Intel Prescott -- at stock speeds -- can push the Load temperature up into the 50's and even 60's Celsius with the heatsink cooler that comes bundled with the processor, and users are encouraged to use heatsinks or coolers from other sources.
"Are there any good temperature monitors?"
ANSWER:
Myriads. Plenty. Many. Legions. Scores.
Your motherboard and processor have built-in sensors, which can be monitored by the motherboard software. With Intel mobos, this would be "INtel ACtive Monitor;" with ASUS mobos, "ASUS Probe." You can also download programs like "Everest Home," "Motherboard Manager 5," or Alfredo Comparetti's "SpeedFan 4.x"
Of course, you can also purchase a front-panel controller that provides some four or more plugs for thermal sensors bundled with the controller. These can be taped on at strategic locations to components -- such as the underside-center of your graphics-board processor between the GPU solder-pins. Or at the bottom of the CPU heatsink -- as close as possible (but not between) where it mates up to the processor cap. Or to the top of a hard disk. Or to the heat-spreader of a memory module. Or the bottom of the heatsink on the NOrthbridge chipset.
Some companies even make temperature monitors that do not provide fan control, and are therefore that much cheaper.
Usually, a thermal-sensor taped to the bottom of the CPU heatsink will show a few degrees lower temperature than measured by the processor's own sensor. This might provide a guideline for interpreting temperatures sensed from the bottom of a chipset heatsink, since (I don't recollect or know of any) motherboards which monitor chipset temperatures. Usually, you can only monitor the CPU and an "ambient" or motherboard temperature from the mobo.