You should do a little bit of research in terms of looking at what the mandatory courses are, and what courses you want to take.
I would take a look at these. You're probably most interested in the first one, so take a look at the program requirements for level II and above.
You'll notice (as Ooburii said) that the mandatory courses include a cognition course, a neuroscience course, and an animal behaviour course. The others are just complementary courses that are meant to give you a good subset of skills to take with you throughout the rest of your academic career.
For level III and IV, you have quite a bit of freedom in terms of fulfilling your requirements. Take a look at the 3rd and 4th year courses, and see if any of those interest you enough to go into PNB. Here's the link to the courses. http://registrar.mcmaster.ca /calendar/2011-12/pg2305.html
In terms of the future of the program, I was in psych before they had the whole PNB separation. I took one of the second year PNB courses and was able to see how it was organized compared to previous years. In my opinion, I believe the changes were a good thing for PNB students. The program is much more organized that it used to be, and you actually get to know the people in your cohort since you're taking many of the same classes together (at least for second year). There is a lot more structure in it too, and way more guidance than we were ever given.
Bottom line for any program that you're thinking of choosing, take a look at the program requirements and see what courses that fill those requirements might interest you.
Good luck making your decision!