What is the “Quick and Dirty” way to transfer WordPress installation?

It’s all in the docs: read Moving a Blog – WordPress.com and Move to Self-Hosted WordPress – Guiding you from WordPress.com to a self-hosted WordPress site.

You have to do an XML export before you repoint DNS to your new host siteground.

And, you may have to choose new plugins on self-hosted, as well as a different theme, though you can download .com themes here via svn and adapt them to self-hosted: http://themes.svn.wordpress.org/