As mentioned already, there is no need for any formal ground school for the FAA PPL. The question bank for the theory exam is freely available and there are plenty of sites where you can practice the exams prior to going to the USA. Here are a few of them;
Practice Pilot Exams
FREE Pilot Tests Prep Center
The private pilot practice FAA exam site. The definitive guide for pilot training
When practicing the exams it is nice to have a print out of the accompanying handout to refer to rather than having it on the screen (especially for the map related questions). You can get a copy of it here but, be warned, you will use a bit of printer ink :-)
http://www.faa.gov/training_testing/...CT-8080-2E.pdf
When you go to do the skills test there will be an oral test before hand. The length of the test is at the discretion of the examiner but count on somewhere between 45 mins and 2 hours. The examiner will really focus on areas of the theory exam where questions were answered incorrectly so be sure to brush up on any wrong answers in advance. There is also a handbook from ASA - The Private Oral Exam Guide - which covers pretty much everything that can come up in the oral exam. When you get to the states you can pick up a copy of that and study it.
Hope this helps.