How much bandwidth do you need between buildings? Is there clear line-of-site between the two? Any trees, etc. that grow and block line of sight? You have rights to mount antennas, etc. on top of the building?
You can go anywhere from T1 speeds all up to 100Mb/s - It just depends on your budget. I used a 100Mb/s wireless link over about 2 miles and it worked like a charm. It supported about 200 users, but cost about $80K. There's a lot of smaller stuff out there, however, that can be purchased a lot cheaper.
- G