MyISAM is fine - unless you write to a busy table. If that happens, table locking will definitely kill you. In my experience, the InnoDB performance overhead is not an issue, but of course this will vary according to your specific situation. And also of course, InnoDB has other things to consider (need to be smart about the tablespaces, since they won't shrink automatically.)

Don't know Sphinx or what you'll use it for (sounds to me that it's useful for the site's search engine.)