Hi Arun,
I would like to recommend G-Etherenet between buildings with fiber if you can. If not wireless line of sight using 802.11b with spread spectrum.
I would also recoomend that you use Vlans to separate out your high priority real-time Voice traffic from your routine e-mail and HTTP traffic. Please looking a policy based system for bandwidth control and policing.
Feel free to write me for vendor recommendations
Yours truly,
Rick Gallaher