The GACE is the test (or series of tests) that teachers in Georgia have to pass to be highly qualified. The PRAXIS is used by other states.
Laurel Burdette has designed a wonderful study guide for the Praxis science test which I have stolen and adapted to several GACE tests I have taken. She deserves a lot of credit for designing something that is slap-my-head simple and useful.
If you are given the standards that will be on the test, you start with those. Then that is the framework for building your study guide.
I have found this to be very very useful!
*** edited *** SMiller suggested that Brightstorm has science videos on a variety of topics