Go buy a study guide for the GMAT!! Buy one that comes with a CD. I bought a study guide with a CD, published by Kaplan, and it helped tremendously. The cover price for this guide is $37.00. The CD had all the instructional material as the book, plus tons of quizzes and four full-length practice tests. It really gets you prepped to take the real thing. It's timed and if you don't have a decent pace, you could very well end up not answering a lot of questions.
The GMAT is an adaptive test, meaning the better you do, the harder it gets. Every correct answer gets you a harder question. Add that with the timed aspect and it gets very hectic! All I can say is buy a guide and spend some serious time on it.
Factor in that you have to pay good money to take the GMAT while the ASTB is free...
Hope this helps.
Any questions?