Originally Posted by
zelah
the best bet is to change everything (cog/chainring/chain at the same time) because when they do wear out they wear out evenly. switching things separately does lead to noisiness
Hmmm... If replacing the cog then definitely replace the chain, as mentioned, the two will wear out together meaning the chain will stretch out and grind the teeth in the cog to account for the stretching. As always, Sheldon has a good article on this:
http://www.sheldonbrown.com/chains.html#wear
As for replacing the chainring as well, I'm not too sure about that one. The article doesn't mention chainrings specifically and it might get worn less since less force is applied per tooth. Not sure though.