What NAS OS do you plan on using? Openfiler? FreeNAS?
For the additional $8, upgrading to the dual core Wolfdale is a good idea, unless space/heat is an issue then the 35W single core is a better choice. Both of those OS's will run without issue on either the single or dual core, especially if all you are using this machine for is to be a NAS/File Server.
I highly recommend FreeNAS.