Hi,
Kindly use the Cisco Feature Navigator:
http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp
NOTE: With some IOSs your router won't have enough flash, if so load the router from tftp (i've faced this case more than once).
I've used c2600-j1s3-mz.122-15.T12.bin before.
HTH, please do rate all helpful replies,
Mohammed Mahmoud.