nostall :
MX 4 and Arctic Silver 5, along with Tuniq TX2 and TX4 are some of the best thermal pastes out there. Andy is correct, buy a small tube; it will last through 4-8 thermal paste replacements. Directions can be found several places including here on Tom's as to how to do the job properly. Here are a couple good articles re. various pastes; the Hardwaresecrets one is informative and entertaining:
http://www.pcgamer.com/geek-tested-17-thermal-pastes-face-off/2/
http://www.hardwaresecrets.com/thermal-compound-roundup-february-2012/5/
Andy Chow: Might want to look at this article. Your information is not correct re. Arctic Silver.
http://arcticsilver.com/as5.htm
Damn, you're correct about Arctic Silver 5 containing silver... I thought all paste containing silver was conductive, but that same link says "Not Electrically Conductive:Arctic Silver 5 was formulated to conduct heat, not electricity."
Learned something new today.