About GoT and ASoIaf. NO SPOILERS

A man has a question

As of now, I am about halfway though A Clash of Kings. More specifically, Arya has just been brought to Harrenhal, Stannis is attacking Storm's End, the Greyjoys are planning to invade, etc. So my question is, once I finish this book, how much of the show would I be able to watch? How closely does it follow the novels? Does it diverge from them? I was planning on trying to read the whole series before diving into GoT but damn, it will take awhile for me to read the next three books. By that time, there will be several seasons of GoT to catch up on. Though no matter what I decide, the sixth book still won't be out!