The nice thing is in the given scenario you have options. Your options are swayed by many factors however the three MAIN ones are:
1) What is your budget?
2) How critical is security?
3) How critical is optimum speed?
If you have deep pockets and you need speed and security...
You want to go the route of a fiber back bone. You didn't mention the distance between buildings which is a very big detail you leave out. IF the distance is great you can lease lines from your teleco but this will incur expensive to are you Friggin INSANE!!! kind of monthly charges (Bandwidth across the WAN isn't cheap).
Of course if your buildings are only a 100 feet or so apart (even this can be expensive, that's why people don't like doing long fiber runs they are EXPENSIVE) you can have fiber lines trenched into the ground. This would be your fiber backbone and you'd consult with a professional datacomm cabling company to do this job. They know how to install it to required specs and any codes. THIS WILL NOT BE CHEAP! (Warning you now).
However this will give you killer bandwidth between your buildings, it will also be invulnerable to EMI.
Go cheaper and if speed isn't as important there are many other ways to connect your buildings.
You could go wireless - you won't get 1 gb going wireless unless you want to get a license from the FCC (if you are in the states) and don't mind plunking down the equivalent to $10,000 - $15,000 US. The good news is you can get 54mbps from off the shelf products.
Of course this is just pecking on the surface.
You still need to secure the connections with IDS, Firewalls, strong policies, etc. You'll need to do even more with wireless security.
Just trying to get the wheels spinning.
PS. People do this kind of networking all the time, its not that hard -- you just need to know your finances and more importantly your NEEDS.
