If you are not yet sure which research area you will choose,
then it is advisable to take courses that provide a useful
foundation for most areas. For example, CS 473G (Algorithms)
is an essential part of virtually all of computer science.
A graduate-level course in computer systems (hardware,
software, or networks) is also advisable.
At the same time, it will be helpful to take courses that
help you to understand potential areas of research. Talk
with faculty in your potential areas of research interest
to make sure that you are building a background that would
not lead to difficulties in the PhD Qualifying Exam.