This question is more of an SEO question that one of domain names - but to answer your question a 301 'Permanent Redirect' is the best solution. (Personally I would try to do this at the page level rather than at the site level - so if someone has bookmarked the page on your old site about blue widgets they land on a page on the new site about blue widgets rather than landing on a the home page.)
With a 301 redirect, everyone will be redirected to the new domain. I suggest when you 1st do this, you do not change any site titles, content, or URLs. You do not want Google [or the other SEs] to think this is a new site, want you want is to transfer all page rank/weight to the new domain. After a couple of weeks, you can update the new site.
[edited by: TowerOfPower at 5:20 pm (utc) on July 31, 2008]