newbie question - delete before insert

Hi,

a question that I imagine is a faq but have not been able to find much help.

How do people implement insert or upate ( otherwise known as upsert ) behaviour in postgres i.e. insert a row if it's
keydoes not exist in the database else update the existing row? 

I tried using an insert rule to delete any existing rows first then insert however this leads to infinitely recursive
rules( which postgres properly rejects. ) 

I'm guesisng that the most sensible approach would be a stored proc/function?

Cheers
G.