if the fan is on the heat sink then its a hard call.
Dells use custom Mobos as well as custom heasink, so to get a new heatsink you would have to replace the mobo, but a new mobo wont fit in a dell so a new case is in order as well. you can try and do a custom joby on the Heat sink and butcher on a new quite fan but it would be meatball surgery.
if you have a black dell Dimention youll find the the fan is a vented on straped to the back of the case. this can be replaced but again it would be a case of meatball. but far lass so than doing it to a heatsink. i went through somthing simmilar when my old dell overheated, now the only part of the orignal left is the gcard which ill replace soon.
if you decide to get new parts this is a exilent site for silent things, every thing on there is certain to be quite.
hope this helps