Thank You everyone very much for ur replies.
Atmega 8 is no issue. Atmega 32, 16 n 8 differ only in their memory size & speed of processing. Okay I shall use Atmega 16 or 32 then. My development board is AVR development board and not just specific to Atmega 8. I saw this robot on a video on Youtube. The link is in my first post. I checked the uploader’s channel and found out they used SIM300 and AVR atmega 16. Actually I have seen people making projects like remote SMS lighting system where they switch on a bulb & fan by just sending an sms. They used a relay ckt too which I shall replace with L293D motor driver IC. I modified that concept to replace the bulb by LEFT SIDE MOTOR and Fan by RIGHT SIDE MOTOR of my robot so that my robot moves by differential drive. I have actually modified a line follower program to suit this project.
Thank You Sir.
Soubhik.