I took the GRE about a year and a half ago (fall of 2000). I looked at the sample material on ETS's website. Thats all the studying I did, and I did *OK*. I didn't do as well as I had hoped, but I did well enough to get into all of the graduate schools I applied to.
What is the GMAT? What do you plan on studying in grad school? I would recommend getting a book/software to help you study instead of, or at least BEFORE, dropping $2000 on the study classes. Get the material and take a few practice exams. If you can't get your scores up to where you want/need them to be, then drop the cash on the classes.
Ryan
PS- One more thing to remember. The GRE and other standardized tests are only keys to help you get through doors. An outstanding score may help you get into a school and/or get a fellowship, but what really counts in the long run is your performance at the school. Once you are in, no one will look at your GRE's again. So, if you don't do as well as you had hoped but you do well enough to get your foot in the door somewhere, you are set. It is up to you at that point.