Hi Imran,
The minimum router that i've used was a Cisco 2611 with 64M DRAM running "c2600-j1s3-mz.122-15.T12.bin".
Please use Cisco Feature Navigator to find the appropriate IOS image to support the features you require on the platform you require, but please do note the required Flash/DRAM.
http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp
BR,
Mohammed Mahmoud.