Noz03 on "Changing permalinks"

Im trying to change the permalinks from the default on my very new wordpress (20 pages so far). First of all, what is the best type for SEO and other purposes? just postname? Or category/postname?

Also, after I change it in the settings, do I need a plugin/what plugin do I need to update all the links/etc? Last time I tried it none of my pages linked to each other any more :(

Thanks for any help!