‘Should you’ is a hard one, they are not necessary as the inbuilt monitor on P4 and AMD chips will shut it down if it overheats, I do like it though, gives you a better idea of what’s happening.
Most (maybe all) new chipsets/moboÂ’s/processors already have temperature monitoring built in, I have tried a number of windows based temperature monitoring packages, and none of them tell the truth.
If you want to know the true CPU temperature, check the BIOS.
Otherwise there are a number of companies who make temperature sensors, Vantec, Lian-Li, etc. most are coupled with fan controllers and high temperature alarms, I have the Hardcano 12 from Thermaltake and would recommend it.