I think GMAT 'practice' books are tougher than the GRE 'practice' books. But hey, the real GRE Quant is much tougher than all practice materials. Keep an eye on the Just Finished GRE forum and keep practicing whatever material you find. I also suggested before to view the GRE Mathematic Subject test. Most of the questions are manageable for Eng/Sci students. About 20% of the questions can be answered on the light of GRE basics.