I watched seasons 1 and 2 before starting to read the books, so I knew most of the action in the first two books before reading them. And I have to tell you, it didn't diminish the experience at all. Now, I will say that Book 3 is absolutely spectacular, and I'm glad I read that one before seeing this last season, but I still think it will be very much worth it to read the books even after seeing the first three seasons. I'd definitely do it -- GRRM is a really good writer.