If you already had a registered domain name associated with the blog here, then you would not lose any search engine ranking, but if you are talking about this blog, http://musicheart.wordpress.com/ , then you will be starting again from scratch with the search engines and it will take anywhere from 6 months to perhaps as long as a year for your new blog to get back to where the one was.
The content here will stay here unless you delete it or delete the blog. I always suggest just setting the blog here to private instead of deleting it since if you delete it, you will never be able to get the same URL here again. The blog here is yours forever. It will never be deleted by staff no matter how long it sits unused.
There is a trick that can be done with the domain mapping upgrade that will automatically redirect people coming to your blog here over to the new blog and it is covered in this support document: http://support.wordpress.com/domain-mapping/redirecting-your-blog/ .
If you do not do the redirect, my suggestion is to edit all posts here, mark them as private and then make one final post here telling people you have moved and giving them the link to the new blog. Also, go to settings > privacy and select the second option that blocks search engines. Watch the stats on the blog here and when it gets to a trickle (6 months) then set the blog here to private (settings > privacy) so that it will not compete with your new blog.