Is it possible to implement MVC pattern in an existing project? If yes, what are the steps involved?

Your existing project uses WebForms with CodeBehind for event handling I assume, even though it has an MVC "approach"? Although you can use WebForms and server-side controls in ASP.NET MVC  the whole concept of postbacks and event handling disappears, so you can't really do a direct port, no. I also don't favour using server-side controls even though you still can.

hhariri:

I also don't favour using server-side controls even though you still can.

What else would you recommend? Do you have a specific "client-side controls only" library in mind?

 That bringsup one more question. jQuery or ExtJS? Which one do you prefer?

I like both :). Although I'm currently using jQuery more. Obviously you have the licensing advantage with jQuery and there's also a wealth of plugins available.  

 Have not looked at the licensing but I assume that jQuery is easier to deploy in production than ExtJS.

yaip:

That bringsup one more question. jQuery or ExtJS? Which one do you prefer?

YUI. Although it's light on widgets so I often use the ExtJS library with the YUI extension.

Do I always have to point to http://yui.yahooapis.com/ to get  javascript files?