Another possibility to check for some CPU from that time, which will fit in that socket, but maybe runs faster, if you need a faster NAS. Of course, parts are possibly not new and used, but cheap alternative instead of a complete upgrade. :3
Use a high quality PSU for a machine that's this important, don't skimp on it. Get a name brand gold rated PSU if possible, like the Corsair RMx, Seasonic Focus Gold and Cooler master V series.
The drive config is somewhat dangerous as there's no redundancy at all. I hope you have a good means to backup all data regularly and safely.