What Medievalist said.
I am not familiar with the Columbia English program, but there are some general things you need to consider.
The most important is, do you really want to do this? A Ph.D is a long process, 6 or more years. Even if you have a burning love for your subject, you will change your mind many times during these years. For one's emotional continuity, it is often better not to start the process at all than to quit half-way (IMHO).
If you are dead set on a Ph.D, please make sure your future advisor is not a complete ass. You need to scout the departments for potential advisors (read what they publish, see if you like it). Then check out the departmental website, and write a nice email to a couple of graduate students to get a feel about the program and the faculty members.
Two real-life examples of bad matches.
A certain professor is famous, funny, and a great writer. Every year the dept. admits a number of students who want to work with this professor. Unfortunately, they find out soon enough that s/he has absolutely no interest in mentoring, and that her/his students take twice as long to finish as anybody else.
Another professor offers great support to students, but when job-hunting time rolls around these students find that their mentor quarreled with everybody else in the profession.
English is a big field with many jobs, but there are many great candidates, and from what I understand the job search is a bloodbath. This is something you should probably consider.
Hope this helped,
Rose