^exactly. if the colleges you want look for students with 2000+ SATs, then you should think about retaking them. however, if they're average for the schools you're looking at, then you're fine. SAT scores are really only good/bad in comparison to where you're going.
in response to your question, i've used princeton review to study for the SAT subject test in math level 1, and it got me an 800, so i'd recommend that one though different people have different preferences. i would recommend PR
but i'd suggest going to the bookstore and flipping through a few different books to see which one has a format that appeals to you, then go with that. also, if you're in US history and i guess trig right now, simply going through your class notes will help a lot. by studying hard in your classes and doing some outside review with a book or two, you should be all set. good luck!!