wrap p tags in posts with div class

Hello all

I am trying to create a blog for a client and to simplify things I have it set up so that all images in a post are in a div set to float right, I want to do the same thing with all text added to the post. I have tried adding
jQuery('p').wrap("<div class='post-txt'></div>");

to many different PHP templates in the wordpress core, but all to no avail, if anyone has a way of doing this or can point me to the PHP file that wraps all text added to a post in p tags that would be very very helpful, i’ve been on this a week and can’t seem to work it out.

All the best

Harry