How about checking out ZendFramework and CakePHP as two well developed frameworks and testing them out? Either way, you will need time to get to know them and work with them.
Now, it still could be overhead to use a framework for a simple site like this. If you're developing something like MySpace or Facebook or Digg, then a framework might be in order for you, but for a simple forum or website which holds user data and profiles isn't exactly a very intensive site which would require a framework.
As for templating, Smarty is good, or you can just use good-ol'-fashioned php to do it. Create a header and footer file and then include them as needed, or set up a templating function or class to automatically do it for you.