Call Javascript files in Header or Footer?

I’ve been using JavaScript for a few years now I’m not a huge expert I rely more of JQuery. I am just wondering what best solutions are there to call Javascript files (and to make pages load faster):

<script src=“http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js” type=“text/javascript”></script>
<script src=“_js/jquery.validate.js” type=“text/javascript”></script>
<script src=“_js/jquery.metadata.js” type=“text/javascript”></script>
<script src=“_js/script.js” type=“text/javascript”></script>

Normally I request all these in the Header like you normally would but I have seen and heard people to call their Javascript files in the footer to make page load faster and have all their Javascript functions all in the footer as well.

Is this a good idea? What are the downside? Should I just keep all my JS in the Header as usual?

A similar question was asked recently (by me!) and got lots of interesting relies: