Hey,
Algebra will benefit you most of all, as well as geometry and data analysis. A strong grasp of "general arithmetic" also will suit you...
Some stuff I can remember off the top of my noggin' from the GRE includes:
Exponents
First and second degree equations / inequalities
Simultaneous equations
Writing equations to solve word problems
Estimation
Parallel lines
Rectangles, triangles, circles, spheres (Better have those formulae memorized)
Pythagorean Theorem
Geometry w/ coordinates like slope, intercept.
And statistics (data analysis remember?) like mean, mode, median, standard dev., range, etc.
I highly recommend either the Princeton Review or the Kaplan GRE book! I used the latter, and it really helped give me a really good grip on the material (Since they know the test so incredibly well).
Remember, you don't get a calculator on the GRE... at least I didn't, and it wasn't that long ago. So prepare yourself to do computations in your head if necessary, and simple arithmetic stuff in your head. I found that the best preparation was to be comortable (not sweating bullets) with all high school and fresh/soph year of college application of mathematics.
Hope this helps.