It seems my previous post has not come through yet.
The idea is that it will share large files (from 500mb upwards, mostly data files, videos etc etc.) between about 3-5 computers. The principle is that if computer 2 needs an updated file1 then it is immediately availible on the server, no waiting time requiered. The changes will be saved to the server on the fly, hence the 4 hdds in raid 10 (for redundancy ).
I figured that a dual core will be fine for these tasks. It is going to run windows home server (have not considered linux).
Yes 16gb is probably overkill - 4 is enough.
And, since it is reachable via network, you do not need any silent models, just put it in another room and a long cable will do.
Good point on the NAS - they are much cheaper (by a factor of 2 at least), quieter (and more fit for purpose) than I originally though. Can NAS boxes cope with what i have described?
Thanks for your help