Many people have probably told you how important the GRE is. Unfortunately it is true for most graduate schools. However, check to make sure it is true where you want to go before you go any further.
Forget the $1000 plus prep class unless you have money to spare (in which case you should not be looking here).
Buy the Princeton Review book "Cracking the GRE" off of Amazon dot com. Currently listed at $22 or $14 depending on whether you want the DVD which is probably worth $8 more.
Read the book and do the practice problems and tests. The class basically just reads the book to you. Is that worth more than $1000?
Once you feel you have it down, go to the ETS.org (they are the ones that make the test) site listed below and download their free Powerprep software and practice some more before you take the real test.
Relax and eat and sleep normally before you take the test.