Hi Shahnawaz,
With so many versions of IOS possible for your router, you're going to get as many different IOS versions as there are people answering your post.
If you hit this link (you'll need Cisco.com access):
http://tools.cisco.com/Support/Fusion/FusionHome.do
You should be able to walk through the questions and narrow it down to a viable IOS version (I'd suggest something in 12.2 or maybe 12.3 if you have the RAM).
If you could upgrade your RAM, then your options are much more open.
HTH
Steve