Thanks for your reply.
My unit is running a E2160 at 1.8GHz, with a stick of 1GB DDR2 ram.
I've installed Plex and it's amazing. But if multiple people are watching, it might struggle. I have also read and want to try out transcoding with the Ultra 6 Plus. That's why I think I would benefit from a speedier CPU. Plus, the 45nm parts run cooler idle, and draw less power while idle.
My goal is to make the ReadyNAS more quiet than my full sized tower computer, think that is a resonable task. (It has 4 120mm fans, plus a 210mm). You are right, compared to the CPU fan, the 120mm is rather quiet (however there is a slight clicking sound). That's why replacing the CPU fan is my no.1 prio. (I thought might as well do the CPu while I'm at it)
I do have a Q6600 just lying around, but I think the 105W TDP might kill a NAS or two