My experience is this: the comfort zone for a CPU varies tremendously with the type of chip; different sockets and even different cores vary widely, and to further muddy things, each CPU is unique in a number of ways- I've seen three systems side by side running the same chip, memeory, etc., operating perfectly happily in three distinct temperature zones. it's not nearly as big a deal as people make it out to be- much beter to judge performance from benchmarks than by temp, and the specs are pretty much unreliable. and, just to make it more fun, the thermocouples used to measure temps are usually wildly inaccurate! hooray!:)
because the die-making procedure is pretty much a 'pray and spray' occasion, and the processes involved are damn near subatomic, the transistor patterns on a single die can vary widely- they chop them up into cores and sort them based on how much juice each little chunk can stand- or what quanity is needed for a particular run- that's why overclocking freaks get so worked up about "CPU from Week 97!!OMFG!!", because that week, maybe a particularly fine die was tested and sold at a far lower clock speed than it can really take.
real world, with modern chips, expect P4's to pull about 55-70 watts( thus temps around 55C-70C., but it's not a huge deal- many of the P4s I've handled recently, like the 3.0e, 3.2e, etc., seem to run just fine at 70c and higher, and most P4 boards don't have thermal event shutdowns until the 80's. perfomance is definitely helped by better cooling, but only up to a point- I don't see much difference ebtween a P4 runing at 60c and 40c. doing wacky overclocking like dunking your PC in a fishtank and getting the CPu close to the ambeint temperature WILL help- it's usually a PITA and far more trouble than slapping some ram, a decent GPU and a RAID array on there.
AMDs are touted as cooler- they are, mostly, and the new AMD64s are practically refrigerators- my AMD64 3000+ at home runs overclocked, full CPU usage, at a smokin' 35-7c :D. Athlons/durons run hotter- , but vary widely- expect 45-65c . as a rule, the CPU temp shouldn't delta much above 10c- huge spikes of 20c indicate a problem.
at home- Althon Tbird, stock cooling, 55-63c- runs like a dream. AMD64 3000+ Winchester 29-37c with a Zalman 7000Cu(rocks!). Celeron 2.2(in a laptop) 65-70. at work- Sempron 3000+- 40c. smoking new gaming machine we built- AMD64 3500+ 55-65c. i just saw a Pentium running at 200MHz with a heatsink that was quite literally too hot to touch. really cranked on that Windows 95, tho :)
desktop monitoring soft: Motherboard Monitor
- not garuanteed(sic) to work with everything.SpeedFan
- shrug- just tried it now, seems to work ok for me.
ok, so that was a long post...