I don't think so. Do you feel them running too hot? I also have two HDDs and I use my PC longer than you do I suppose.
Anyway, it's a compromise between the noise and the coolness. If you can take more noise, then it's better to keep the HDDs cool, nevertheless. If I add anymore fans, the noise level will increase. That's why I'm a bit hesitant.
But with the stock HSF for Pentium D, I don't think you will notice any added noise from fans. BTW, you can run those fans at 7V to keep down the noise and reduce suction of dust particle into the case.