Steve,

I would stay away from NT as it will stop being supported at the end of 2004, which may sound a long time away but this was only an after thought, it was spposed to be 2003. 2k is very easy to administrate with a host of options and upgrades that NT won't have, including support for newer technologies. SBS will be a waste of money for you as it includes exchange, sql server and a few other that you have said you are not interested in.
Dual processors are great for heavy procesing but i don't think that you will see a double the difference effect by getting a dual processor, the money would be better spent on faster hard drives and faster network connections. raid is great but i think that it is slower than normal disk accessing just by its very nature, a simple mirror might be a better idea than full raid.
Future proofing your system with 512 rather than 256 might not be a bad idea, there are faster rams out there now than whrer available when you file server was bought so this may be an area to look at. if you file server takes on another role at this cash strapped time, which may be the case, then you'll be glad that you got the 512 although 256 is plenty with just plain file sharing.
using a gigabit card is ideal but if the pc's are on 10/100 might be an idea to upgrade them, and your switch/router.

Cheer
~M