I'm preparing for entry into a Physics degree course, and was planning on a double major in Physics (with a specialization in Quantum Information later) but am now considering a minor in Astrophysics/Astronomy. How useful would Astrophysics be? I have an interest in that area also.
Actually Quantum Physics is a requirement for any physics you want to do as a future physicist. It is in every realm of study you are going to perform. So if you go into astrophysics, you would see it in things such as degeneracy pressure at the end of life of stars. You would need it also to understand the forefront topics of black holes thermodynamics, since quantum physics tells you how can a black hole radiate and evaporate. Going further in the realm of Cosmology, you need it to understand the matter-antimatter asymmetry of the universe. Finally if your goal is to develop a theory of everything you need it to develop first a quantum theory of gravity. So I would say focus primarily in Quantum mechanics which is a little complicated to understand relatively to a classical theory such as General Relativity.
Also everything depends on what do you like to do your research on.
I'm an Applied Physics and Applied Mathematics double major, and I'm for sure going to specialize in Cosmology and related fields--call me crazy. If you want to go all the way with it, I would personally do Mathematics as a second major since everything you're going to do IS math. You'll HAVE to take various courses in various branches down the line, so you will be educated in the Physics core in topics like Quantum Mechanics, and later down the line Astrophysics. ( Based on looking at Purdue's, Rose-Hulman's, and a few other University level plans of study ). Your grad work will of course be your chance to really concentrate on exactly what you want to do. You'll be able to pick up what you need along the way as well as far as the mathematics for the Physics if you didn't already get it from the math core. The Physics comes much more naturally if you have a fundamental understanding of the mathematical operations therein.