Basic LAN Security – Hardware, Software or Both?

As the only techy in our small organization, I manage our small network. To date, I have relied on the firewall in our router (Linksys WRT54GS) as well as a software (by Eset) on every desktop to provide protection from unauthorized outside users. My question is: would it be wise to implement a new, dedicated security device that could protect my entire LAN so that I can eliminate the software firewalls? My goal is to make interactivity easier inside my LAN (being able to find other computer, printers, etc.). If I’m not crazy with this idea, does anyone have a suggestion for such a device?

My desktops are Win XP and Vista. I also use a Linksys VOIP box for Vonage. I keep shared data on a Netgear ReadyNAS NV+.