Office network and guest network - manage with separate physical servers or a VM on the office server?

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http://serverfault.com – I have a beefy server (dual Xenon & 16GB RAM with RAID5) I use as the domain controller and file server for an office network on a dedicated internet connection. I also have a guest network with wifi access and a few public workstations for guests to use with its own dedicated internet connection. The guest computers are not centrally managed, but I would like to change that and set up the guest network with its own domain, but still keep it completely segregated from the office network. I plan on using the guest network's server for locking down the workstations with GPO, deploying new (HowTos)