you heard wrong then. the stock fan on an nvidia card is the same through all the manufacturers.
i'll put it this way. can you hear your computer now? if so then you will not notice the 9800GTX, its just not necessary or helpful to turn up the fan unless your overclocking.
the only reason i dont answer to your question the way you wanted it answered though is because after looking through newegg, there is no 9800GTX that has an aftermarket heatsink on it. but thats not to say you cant just put one on.
but seriously the stock fan works great.