Note that speed dials on the start page are bookmarks too. So, backing up the "Bookmarks" file will back them up too. However, thumbnails for speed dials are stored in "Bookmarks Extra". So, you'll want to back that one up too.
Another way to back up bookmarks where you can just import them again is to goto the URL opera://bookmarks and use the export option at the bottom. That will save them as an HTML file that you can import later. It doesn't save speed dial thumbnails though.