like hiviz said, if your gona do normal gaming whit normal OC and in a normal temperature room ( around 22-24+C ) a good aircooler will do the jobb.
but if you gona do heavy OC ( from 4,6GHZ and upwards over long time ) mabye water.
and for that you have to have a good ambient temperature, room temp mabye around 22+c and dowm.
where do you live and what is temp in sumer and winter?
and if you want to do a cheep watercooling you can use a closed loop one like corsair's H100, they are coming out whit a new series called H100i witch you can controll from a software in side OS. ( windows ).
if you gona go for watercooling i recomend to start googeling and read up on it to get more info on it so you know what you are geting in to.
it is expensive, and if you do an error ( spill fluid on a videocard and it short out ( your waranty is lost ) ) it can get more expensive.