I took an older version of the GRE, but studied using the Baron brand of GRE prep book. It helped me get a 1480 (out of 1600), so I'd recommend that. Presumably they cover the new format.
I'd say it's more similar to the SAT than the ACT due to the language portion. There were a lot of those book is to library as soup is to <blank> type things, only with really random vocab. I made a bunch of flash cards and practiced vocab all summer as I worked at the drive thru window (with my Bachelor of Science degree, grrr...). The math was a lot of simple algebra and geometry. I had to go back over geometry concepts quite a bit before taking the test.
I'd say 2-3 months is plenty of time. Some of my friends used the Kaplan books, but I got my Baron book borrowed a lot because they thought it was better. When I took the test, the math and language portions were graded automatically (it was a computer test where the difficulty of the test varied depending on how well you were doing, and they based the score on that) and the writing portion was graded later. The computer format was odd, but like I said I took the old test and I don't think they grade it in this way anymore.
Hopefully someone answers who has used the new format, because they'll have better advice! But I definitely recommend the Baron book and would say expect something more like an SAT than an ACT. It's pretty comparable, just a little more advanced.
There are some free resources on the GRE website. I feel like I took some computerized practice tests, but can't remember exactly where. I thought it was from the GRE site, but you might try googling. It could've been some passcode I got from the Baron book.