I took a practice test from Kaplan about 7 months before planning to take the real thing. From my scores on that test, I tailored my studying appropriately. I ordered 4 books from Kaplan: the math one, the reading one, the one with like 8 practice tests, and the overall one. I found that the overall book had too much overlap, so I wouldn't buy it again. But, I found the other 3 to be very helpful. I also bought the Kaplan vocab cards. That, plus the 4 books was $62 from amazon, if I remember correctly. I started studying in May, spent about 2 hours a day studying, took 1 practice test a week, and took the real test in early August (probably about 10 weeks total of studying). I would definitely recommend the Kaplan books if you are a disciplined learner. I would also recommend taking as many practice tests as you can handle. For me, taking a weekly practice test decreased my stress/anxiety on the actual test day because I strictly timed myself at home, so it was nothing different from the habits I had established. Compared to the practice test that I took in February, after studying, my math and reading scores increased 13 points and 12 points, respectively. Moral of the story: I was pretty happy with the Kaplan prep!
I would also recommend downloading the powerprep software from ETS. You get 2 free practice tests, that are identical in format and timing to the real thing. I also found that my scores on the 2 tests from ETS were within 2 points of my test day score. My scores from the Kaplan tests were significantly lower than the ETS scores, so it could be argued that the Kaplan tests are hard and prepared me well (definitely debatable...)
And about re-taking the test... A) Its an expensive test ($180 or something like that). On top of the already expensive application process, try to save yourself some $$ if you can! B) I found the GRE to be pretty tiring on my brain, and honestly, I would not want to have to take another 5-6 hour test, in that tiny computer cubicle again. And C) I don't know this from personal experience, but I've also heard that most people who retake it don't actually improve more than a few points.