The best preparation for the GRE is a lot of practice and being familar with the types of questions being asked. So I recommend picking up a book with lots of practice tests. I’m not sure what your major is or how well you need to do on the GRE but I found the math to be very elementary and the the verbal section is a bit more difficult because there is a lot of vocabulary. Besides doing lots of practice test, I recommend studying the vocab words given to you and being familar with latin words.

For a prep course, I would skip it, esp. if you haven’t taken the GRE yet. You should take the test first and then take the course if you feel that you could use some more tips and advice that you think the books were lacking in. The course would probably only raise your score a few points.