Yeah like the others said the warantee depends on the manufacturer, I couldnt see sapphire actually having a leniant warranty after some of their other shady business practices.... but you may have a difficult time finding a cooler for under 20 bucks.
I have heard good things about Xigmateks
battle Axe cooler, and most people claim a greater than 20 degree drop in load temps (coffee plz no dropping load jokes)
They had a rebate on them a few weeks back, if it returns you may be able to get out for just over 20 bucks.
A friend of mine wanted a pair of these due to his 8800 sli setup running so hot, but the coolers themselves take up 2 slots apiece, and having 8 slots simply for your VGA setup is a bit much
Since you were hesitant before to spend the extra money getting a 4000 series card, Id really just try to improve case flow before you go blowing extra money on card coolers