Not sure I understand your question. Is your site already on the DreamHost servers and you want to make a backup? You can do so using FTP and shell access. If you prefer not to download each file one by one, login to shell and use tar or zip to archive them first. As for as backing up MySQL, if the data is too large to use phpMyAdmin, then use the MySQL tools in shell to dump to text files.
Perl / MySQL / HTML+CSS