Hi Ganesh

You can have a look at Cisco 3800 series routers which is projected as replaceable units for Cisco 3600 routers.

do refer the following link and check out which suits ur requirement..

http://www.cisco.com/en/US/products/ps5855/index.html

You can look at number slots available the kinda modules it can intake and processing speed as well as throughput which it can handle ..

regds