Hi. You have probably found around on the net that you can swap the_content for the_excerpt to achieve this. If not here's how to do it; open up the theme file that displays category posts(usually archive.php), do a search and replace on "the_content" replacing with "the_exceprt" without quotes of course. You should then be good to go. Good luck.

Thanks Mike, That was the option I was reading in the older posts I found and was wondering if it was still relevant...

I ended up going with 'more' tag option when editing the post itself was the easiest way to do this and it also allows me to control exactly where each post cuts off

Good choice, it's pretty flexible that way.