NJgal.... It really depends on how up to speed you are. Quite frankly, I suggest you spend no less than 4-6 months, studying every weekend, and weeknights when possible. This is based on the assumption you want to score in the top percentile. If you've taken the SAT, you know it's all about speed and knowing all the types of math problems and analogies etc from the get-go.
I took a course prep with Testwellnyc, but mine was for the LSAT. The instructor was excellent and I felt that the methods they used for teaching the students how to approach the problems was worth every penny. I know they offer GMAT classes too, and probably a free class you can sit in for. So check it out. But do take a prep class, it helps you practice and become familiar, regardless of who you go with. Good luck if you're taking it!
Syani - my sister did the LSAT. She scored really well. I definately aiming for the top 15 B schools so need a really good score.