sorting by a particular order

i have a custom query that is showing all 2009 tags sorted by grade.

<?php $recent = new WP_Query(“tag=2009&showposts=-1&meta_key=grade&orderby=meta_value&order=ASC”);

i want my letter grades to go in a certain way.
A+
A
A-
B+
B
B-…and so forth till F-

how would i go about achieving that.

i looked into a thing called ORDER BY FIELD, where the code is something like this…
ORDER BY FIELD(grade,’A+’,’A’,’A-‘,’B+’,..rest of values)

my question is, can someone show me how to use that code in my custom query? i tried dooing it myself but it was sorting wrong.

any help is appreciated