first off, any cpu heatsink/fan will fit fine, the zalman cnps9500/9700 are popular ones with people as they will knock off 10 degrees off ur cpu temperature and also comes with a fan speed controller. take a look at newegg.com and just pick which one u like, all cpu coolers will fit as long as they say supports socket 775.
concerning measuring cpu temps, you've got a program on your p5n disk called PCProbe which measures fan speed cpu temps, and voltage. others ones like speedfan and coretemp are good as well, have a look in the downloads section in the forum
the heatsink on the nb will be fine for the amount of overclocking u want to do, i've got my cpu overclocked by about 1ghz and is fine no problems for 3 months, not much change in temperature. i would recommend a heatsink for the southbridge though, as long as its a half decent one thats fine
concerning measuring cpu temps, you've got a program on your p5n disk called PCProbe which measures fan speed cpu temps, and voltage. others ones like speedfan and coretemp are good as well, have a look in the downloads section in the forum
the heatsink on the nb will be fine for the amount of overclocking u want to do, i've got my cpu overclocked by about 1ghz and is fine no problems for 3 months, not much change in temperature. i would recommend a heatsink for the southbridge though, as long as its a half decent one thats fine