Changing URL structure of a site - correct technique for SEO appraoch - 301 headers?

[deusevil]

Ive been making changes to a old site Ive worked on, one of the tasks Ive taken is to change the url structure to something alot more friendly.For example pages currently work via query strings such as \[code\]?action=contact\[/code\] and \[code\]?action=article&id123\[/code\]. I am now changing them to /\[code\]contact\[/code\] and \[code\]/article/articlename\[/code\] respectively.I would just like to ask what approach should I take for the search engines so that they do not think that I have duplicate content.Of course I will support the old urls, for example \[code\]?action=contact\[/code\], but when someone accesses the old url, should they be redirected to the new one? Should that page also be given a 301 header?Thanks.