I'm no expert with vpns but I have delt with them before. I have only done them in linux and on pix firewalls but it should be similar on other devices I would think.
There are several types of vpns, lan-lan, lan-pc, IPSec, PPTP. Lan-Lan vpns use vpn able devices at both ends. Lan-PC vpns will require a software client on the PC that is compatible with the protocol that the vpn router is using (IPSec, PPTP). Windows has built in PPTP client ability, you can just use the new connection wizard to make it. When choosing the encryption on the vpn router you will want to use 40bit if you have windows machines connecting to it. I haven't had luck with anything higher, I think windows just can't handle it.
I haven't had to do a lan-lan vpn yet but I'd think it would be transparent to the workstations on either network. The vpn router's should be able to figure out the routing for you once the vpn connection is made. One thing I have noticed with vpns though is that they don't like NetBios names and are sometimes stupid with dns stuff. You will likely have to connect to machines across the vpn via ip address. If anyone knows how to get NetBios working over a vpn please let us know. I'm sure google could tell me but I'm feeling lazy right now
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