I would suggest FreeNAS here is the link
. I have it running on a P2 box and I can access it with XP, Macs and Linux. It isn't the fastest solution out there but it would be plenty quick on your hardware (since it's not 10 yrs old like mine). It's also small and free so it's easy to try and if you don't like it just try something else. I like it so I never bothered switching.
It supports software and hardware RAID cards and is fairly easy to set up if you have messed around with other NAS's (web interface is much easier than linux, though not as robust). I also use it for rsync backups every night and it has been stable (currently up since our last power outage 87 days ago). It doesn't have robust multi-user functions which might not work too well for what you want, but I love it for easy LAN file serving and backups.