Hi, I suggest you buy one book, go over that thoroughly, then borrow others from library for further reference.Buy the Barron's one for GRE for sure as its word list , maths overview and other strategies are the best....Well, you may need to practice the tests in ETS book too, but borrowing is better.And then , may be you would want to go through the ARCO book for essays.
For TOEFL, I am myself toying with the idea about which one is the best. If you have a lot of time at your hands, you may want to go for the Longman one as I have heard good reviews abt it. If your grammar is sufficiently good , you may just need some CD only.
In the end, don't forget powerprep!