Stephanie,
In my opinion, if you’re under some time pressure and you’re diligently working through the PrepCast units, you could probably skip the PMBOK completely. Cornelius does a fantastic job thoroughly covering the PMBOK material in his lectures, so you’re right, using the physical PMBOK as well may be overkill. I can’t stress enough that you should also be using an exam simulator. I’m not being compensated by PrepCast for writing these posts, so I’m not trying to sell their products. Having used the PrepCast Exam Simulator throughout my entire study plan, I can confidently say that I wouldn’t have passed the exam without it. Another great reference is the PrepCast formula sheet. I used this to memorize the EVM and schedule management formulas, and then practiced writing my “dump sheet” during the first 5 minutes of each timed practice exam. If you’re doing regular timed exams and quizzes, using the PrepCast lectures, and working a study guide, this should be plenty given your limited time available.