File server or NAS

I have 25 users who store all their files on a Windows 2000 Server (2 Ghz, 1 GB RAM, 150 GB Data drive). The performance is adequate, generally no major complaints. The type of files are Word docs, Excel sheets, AutoCad drawings and photos.

This setup is backed up to a removable USB drive daily, but this entire setup is quite old (approx 5 years).

Should I just buy a newer/bigger/faster file server or a NAS? Any comments/suggestions welcome. Thanks, Jay