Until you forget to put thermal paste on your procecessor/heat sink. Not that I did that, but I think thats what Guy Montag might of done.
"might have done" not "might of" you chav
Anyways, I "built" my first PC 2 years ago. "building" is such a strong word, tho. It's more like lego for retarded kids, where no single piece can go in the wrong spot. The only problems I had were more of a theoretical nature: the manual of the motherboard was written for people that KNOW how it works.
Somebody that never ever assembled a PC will probably run into minor problems. I for one was smart enough to unwrap the mobo in the store, go through the manual and ask the guy that sold it to me all the things that still were unclear to me.
Im still incoherently rambling, because the point I'm actually trying to make is the following:
I bought a cheap core2duo back then, an e4600. Not the fastest, but -supposedly- very OC'able. And it's cheap.
Never ever did I change the heatsink or the fan, I just let the stock stuff on there, cranked the speed up by 600mhz per core, and it's happily chewing away on several millions of computations per second day in day out.
After reading up on the subject I found out that -with aftermarket cooling and more voltage- you MIGHT be able to tickle 200 more out, but seriously, if I have to spend for that kind of speed increase, I might have bought a stronger CPU right away.
Same difference, really.
tl;dr:
Probably 80% of aftermarket cooling is sold to people that want to be special
Stock cooling is hella awesome on a lot of products these days, already.