To some extent, in my opinion, INF320 will help you a lot. But with INF420, you have to do extensive research on a bunch of topics. The prescribed book, is very basic on some topics, and then also extentive on other topics. You really have to work hard in this module and Google will be your new best friend. I have spend a lot of hours just to get the some info that will be relevant to your topic, and can be frustrating sometimes. To sum it up nicely. I also did well in INF320, and so far, I'm doing very well in this module, and loving it. Just one pointer, push the assignments very hard. It is going to make your exam prep so much easier and will enjoy the module so much more.
By the way, do you know when we can register for honour modules for next year?