I don't know the iTunes requirements for using the Store but it would not surprise me at all if iTunes 8 no longer works with the iTunes Store. I know my iTunes 7.5 stopped working with the Store several years ago. I hear they recently changed how the Store appears in iTunes which suggests they did a software change and probably didn't even consider anybody would still be using iTunes 8. Apple is pretty fast off the mark for abandoning support for older platforms and I would be surprised if iTunes 8 continued to work with the Store for this long.
iTunes 9.2.1 (the newest that will run under Tiger but I don't know if even this will still connect to the Store) - http://support.apple.com/kb/dl1056