This bothered me too for a while. There were people telling me to sit f7 before f8 but then there were people who told me that f3 knowledge of FSs was enough for f8 and there is no need to sit f7 before f8. Well, finally I have decided to sit them both in one exam session.
@ahal: You just need to learn F3 for the basis knowledge and take F8. In case you are exempted from F3, I think it is better you may take F7 and then F8