Hi Ramchi,
refer to this page http://www.gsmworld.com/technology/gprs/class.shtml
Usual GPRS class available in todays handsets is class 10 and it cannot go beyond 48Kbps (k bits per sec).
My rconnect goes upto 165 kbps...usually maintains about 70-80 kbps in night...and 30-40 kbps during day.
for GPRS 48kbps is the peek...it will give you the best average of 25-30kbps.
Moreover I do not see any tariff of Rs.250 unlimited internet from Airtel.
Ashok