GRE exams for those seeking a Masters in Computer Science
How well do you have to do on the GRE?  That depends entirely on how much weight the admissions committee gives to GRE scores.  Some universities may not really care.  Others may use GRE's as part of the screening process.  Universities have not adopted a universal policy as to how GRE scores should figure into admission decisions.
It doesn't matter if you are a CS major or any other major -- general principles of good study techniques for the GRE hold true for everybody.  If you are an independent learning who is self-motivated and skilled at structuring your own study, then you can probably just get some good test prep books.  If you need more structure, then you might benefit from paying for a course.