I suggest going an entirely different route. Screw probability models altogether. Learn a good simulation program like statistics101 (
www.statistics101.net) or R (
www.r-project.org). Both are free.
You can learn about the re-sampling method (for free) here:
http://www.resample.com/content/text/index.shtml
Then just simulate your situations to find the answers to the questions you have. You will find that you rarely choose the wrong model and don't have to memorize any equations or probability theory whatsoever.
Sherman