Hello!
I was in a similar position earlier this year when I began thinking about applying for PhDs in Islamic Studies. I have an undergraduate degree in Arabic and a Masters in Anthropology but I have never taken a Religion class or a class on Islam. Don't waste your money and time applying if you won't have a chance of getting in. You will be competing against top students who have an MA and an MA thesis in the field. I have since decided to apply for several Masters programs in Religion with a focus on Islamic Studies. My interests are Shi'a history, ethics and the sermons of Ali ibn Talib.
I applied to: Duke, Harvard MTS, UToronto, Hartford Seminary, and UChicago.
First of all, you need to pin down your specific interests. Then, search for professors who match those interests. Then, email graduate students in those fields or attend a professional conference to get advice on the ins and outs of applying.
Basically, Islamic Studies can be divided into 2 tracks: (1) a Near Eastern Studies department (NELC) which is more the classical, Orientalist tradition which emphasizes heavy language study, historical approaches, archaeology, etc. (2) a Religion department, which focuses more on questions of theology, doctrine, ethics, etc. They are more open to an interdisciplinary or sociological approach which is what I decided to do.
You will definitely need at least 6 semesters of Arabic before applying to a PhD program, so a Masters will give you time to do that. It will also allow you to narrow down your interests to one dissertation topic that you will have to propose. It helps if your MA thesis is on the same or similar topic!
Feel free to message me for more advice.