My school drove me CRAZY with these tests. I seperately took an ACT class, a practice ACT test, and the ACT. Then 7 months later the school says, "HEY! we're making all juniors take a practice ACT, a PLAN test, and we're going to waste an hour and a half of your time explaining the idiotproof directions and "how to read the scores" so even though you're almost 18, you don't have to do anything for yourself yet and we'll just hold your hand" ahhhh!
sorry for my rant but yes the PSAT is practice for the SAT. the PLAN test or practice ACT tests are available for ACT.
MOST schools accpet either and/or of the tests but make sure by going to your schools site. They both test you on general things, math, reading, english, science (actually not science, science is just hey can you read a chart)
However, I love the ACT because you can guess and not get penalized. On the SAT, you get 1/4 a point for every wrong answer. Doesn't seem like a lot but when there are 100 questions and you get 16 wrong thats 16 + (1/4)(16)= 4 so you would have 20 wrong. When you convert the scores to a larger number with more questions... thats a lot more off then what would have been. The ACT doesn't do that.