If it is a simple file and print server: any post 1998 hardware that you can lay your hands on.
I run my home file server on a PII-300 with 192 MB RAM. Be prepared to install a headless station (without X GUI) on such a system but since you have Unix experience that should be no problem.
However, given the increasing energy prices you might want to look into an ultra-low power system since a file server is on-line 24/7. Check mini-ITX systems
www.logicsupply.com. Purchase price might be higher but it pays back.
jlinkels