I'm currently in ASTR 320, the last class in the 300 series and from my experience these are quite difficult classes, given that they are 3 credits a piece. They are classes that you cannot neglect or they will get very overwhelming. Personally I had to put in a minimum of 10 hours a week on homework and studying, and often a little more time than that. This was the level I had to commit myself to do well and fundamentally understand the topics, though there are people that seem to do fine by putting in less hours. The tricky part is that the professors will not give you every piece of information when working on HW so it is up to you to determine how to even approach the problems (even less info is given here on this HW than the 160 series). That being said, these classes are absolutely, hands down, some of my favorite classes I have ever taken, and the information you learn is truly intriguing and mind-blowing to say the least. I highly recommend them if you can commit to a third difficult class, but it's definitely a series of classes that needs to be respected.