Well,if you buy cooler in other shop,how da heck will they know you put it?If you get problems with it you will tell them u used stock cooler

As for Gpu,problem is there arent many coolers(that are good)for such a strong cards.This is the one i found but its not fully compatibile with gtx285 but it can be still mounted and will work better then stock solution.However,for cards like this people mostly go for water cooling as it is much better then air one,but is quite expensive

But anyway,I know your coolermaster HAF will do big job in cooling it,so at begining i think you should try and see how temperatures will be with it,so if they are good enough,you might not even need aftermarket cooler