The 301 will take care of duplicate content - and some of the popularity will pass to the new pages. The listings for the old site will eventually drop out. (Contact all the sites linking to the old domain and request that they change to the new domain.) stephenspenser.com has a good article "Redirects and SEO Best Practice".
I've found that the age of a domain is a big factor in results (all other things being equal) so if your current site has been around for awhile and is ranking well, you'd need a really good reason to make the change.
I've just done this with a site and it's easy to overlook little things, so plan the move very carefully. Use Google webmaster tools to look for problems finding pages and see what pages are indexed.