If your NAS is doing the bare minimum of acting as a simple network storage device then you do not need much under the hood. I'm re-purposing an old AM3 socket (pretty sure it's an Athlon of some sort in there, haven't actually checked yet) and starting off with 4GB of RAM for mine. If you have been watching the NAS videos on LTT that I have been watching then the reference is totally overkill for most simple applications. Virtualization would end up requiring more but as you step up the list of things your server can do then the hardware recommendations gets too long to list here.
Keep in mind I am building a basic home-based NAS that will not get stressed too bad. If you are talking about an enterprise-grade server then yes, the hardware requirements are technically higher than what has been mentioned here. What exactly are the requirements for your server? If you have the budget to go with a 3900X then I figure you could probably rearrange funds to better optimize other aspects of the build (the actual storage, for example).
