In India, Airtel mostly operates on 1800 Mhz range. I personally Prefer (and have) Airtel.
www.airtelworld.com
www.hutch.co.in
You can easily get a pre-paid card and recharge as you need it.
Just keep copies of your passport.. you will need them to get the SIM.
coverage area is usually the entire state and anything outside the
state is roaming.
Roaming charges are not a lot.. just about 2x the in-state calls..
but your incoming calls will also be charged when roaming.
(about 10c per minute or less airtime) GPRS is available
but I dont have any personal experience with it in the area.
Calling International is approximately 30-50c per minute
Coverage is excellent almost everywhere in India. If you see cars
in the area, chances are you have coverage.