graduate work won't affect your science GPA, only undergrad courses do.
and since you already have a pile of science credits, 12 more aren't going to do much for a 3.4
you're fine. only do the grad work if you really want to (genuine interest), otherwise work on something worthwhile such as a substantial EC.
edit: you were talking about a SMP, right? no, don't. No one has a genuine interest in those outside of going to med school. if you really want to do a bio M.S., go ahead, but don't do it for med school.