The importance of your DAT (Dental admission Test) is going to be relative to the schools to which you apply. Take a look at your intended school’s average DAT scores in the past few years. You can find this information in the ADEA Official Guide to Dental Schools.
There are a number of study guides for the DAT, including the Kaplan DAT with CD-ROM, 2007-2008 Edition. While Kaplan is probably the most well known, there are a number of other very good study guides. Barron’s How to Prepare for the Dental Admissions Test is also quite good.
During my own preparation for the DAT I was very satisfied with TopScore Pro for the DAT. It contained 3 complete exams that are in the exact format of the DAT. It appeared just as if you were taking the exam, except it includes a small timer in the corner to let you know how your pace is doing compared to where you should be in the exam. With that timer you are aware if you are ahead or behind in your timing.
I might also suggest taking a look at the How To Ace Your DAT blog. You can find some good tips there to help you improve your score.