Setting up redundant file servers

I've set up a Linux file server for the municipal offices of my town (as a volunteer consultant and Linux evangelist). It's working fine -- Samba makes it look like a Win machine to the rest of the world. But now I'm interested in setting up a second machine as a redundant backup to the first one. The intent is that if one goes down, the other one will take over with little or no lost data and the shift will be imperceptible to the network users. What software should I be looking at to accomplish this? Paul