It is really best to use an aftermarket thermal paste, you never know what grade of paste that has been applied on the heatsink, and it can be a pain to have to remove the heatsink and apply it later cause you find that the pre-applied paste is not any good, and does not cool the processor as best as it should.
You should consider picking up a low price aftermarket cooler like the Hyper 212+ to go with that processor. The stock cooler really doesn't support overclocking much at all.
That's a really good idea,, do people still use stock heatsinks?? i have a hyper 212+ EVO and it's great..... I can overclock my amd phenom 965 x4 to 4 GHZ with this cooler, and it barely breaks a sweat.
It is a bit of a pain to install it though.