It really depends on how well-prepared you are to take the test. For someone with a decent math ability who is comfortable with written English, a 400 is fairly easy to achieve--540 is the average, and many of the students in my courses have incoming scores in the 500s and 600s.
However, if your English is not strong, or if it has been a long time since you have had to do algebra, it can take some study to get out of the 300s. So do yourself a favor--download the two free GMATPrep exams from mba.com and take a practice exam. Take it under exact test conditions--one hour for the essays, 8 minute break, 75 minutes for quant, 8 minute break, 75 minutes for verbal. As long as your score is safely above 400, go in and take the GMAT and don't bother to spend a lot of time preparing. If your practice score is below 450, start studying. You might want to start with our Foundations of Math/Foundations of Verbal books.
Good luck!