As crazy4gmat says, the PR test weights the harder questions...it is adaptive like the real GMAT and it's pretty accurate.
Yes, do the AWA prep! I didn't do any AWA prep for my first GMAT because I do a lot of writing for my work. While I got a 6 on each essay, I petered out quickly during the rest of the test because I wasn't used to doing the entire thing. You need to build up your mental stamina and be used to the marathon before your real test.
Edit: You can't compare Powerprep scores to current test scores, actually. The old Powerprep test is too easy compared to the current real GMAT and GMATPrep. They also made some changes to the scoring algorithm from what I understand, from when they switched vendors. Take a GMATPrep test and see how you do, but don't do that until you have a better understanding of what you're doing wrong and how to fix it. You need to do a lot of studying and untimed practice in between your timed tests.
And crazy4gmat....shouldn't that be crazyFROMgmat? I know I have become that way.
