I can't see nay benefits upgrading from 2 E5440 to 2 e8400. If anything i would think oyu will loose perfomance, you are halfing your cores and ok gainning a tiny bit of raw speed. Personally i can't see the point. Unless there is a veryspecific reason that you want the e8400? usch as an instruction set or soemthing?
I would aslo say that in the long run i would think the e5440 would be more reliable, I have no proof to back this up though.