You would likely not get screened out solely on a 3.4 science GPA...much depends on the rest of your application. BUT, if you can improve your application (GPA and in other areas) by waiting a year, then by all means do so. You should apply with the best application you are capable of. Don't rush things. A good application is better than a fast one. And remember, even with a great application there are no guarantees that any particular school will accept you or even offer you an interview.
What does the rest of your package look like? Overall GPA, MCAT, EC's?