Like you already said, you'll have to review up on Hamiltonian (and Lagrangian if you haven't covered it yet) mechanics, but these problems usually only make up a very small fraction of the test so you should be fine. The biggest areas you'd probably have to study are thermodynamics and optics; together these take up about 20% of the test so they're pretty important. That's really smart of you to try and take the PGRE before the craziness of junior and senior year begins, so if you think you can get optics and thermodynamics covered in time, I'd definitely recommend doing it. Plus if you happen to do poorly, it's not the end of the world, you would have the chance to take the test again, and I'd bet that most admissions committees would look kindly on a poor score from sophomore year. Of course, this is just my opinion, I'm sure that many others would recommend that you wait until you have had all the requisite classes. To be safe, if you don't mind wasting a bunch of money, you can always sign up for the test and if you don't feel like you did very well afterward, you can cancel your scores.
I don't think you need to worry about the scores expiring. If I'm not mistaken, the scores remain valid for five years.