Hardware VPN solution?

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http://serverfault.com – I have following situation: I have three clients in 3 different cities. They are all 24/7 available computer machines running in different schools and collecting energy measurements in the buildings. All three need to securely communicate with remote central database system (in 4th city) and periodically upload the data. I have the information that hardware VPN solution needs to be implemented here, but have no specification about what routers (if any) are installed on the network where database is. I also have information that DSL internet connection is available at all 3 remote measuring lo (HowTos)