Don't try to do it that way. I've been there, and it's pure horror. You should restore the database again, making sure that the target database and its tables are properly collated, and that your mysql connection charset is set to UTF8. You might have to find a way to explicitly state that on some servers, I've run into this problem a couple times but I'm so slammed with other issues today I can't hunt it down for you.
If the table creation lines in your database backup have incorrect collation you can change them to utf8 in a text editor before you begin the new restore.
EDIT: If your database backup itself is corrupted with the garbage characters, and you still have access to a copy still on the server, you should copy it into a new database on the server that uses the correct collation before taking a new backup (such as when doing a server move, you'd still likely have access to the old database - if it's been trashed on the server and the corrupt backup is all you have... try this next idea...)
is an excellent method of restoring databases, and you can define the connection charset in the configuration section of the file. This gets around some weirdness I've run into using various versions of phpmyadmin.
Failing all that, if you google the problem you can find some character filters, and you can use those to replace the garbage characters locally, before you restore it. It's faster and easier than trying to do it in phpmyadmin, as you're just running preset find/replace operations on a text file. They're not perfect, however, and you'll still have the truncated post issue...