Originally Posted by James Sell
Slow down just a little, please. When I wipe all of that stuff, what do I end up with on my phone? Do I lose my contacts? Do I lose my pictures, music and other stuff? If so, how do I save them so I can add them back on to the phone? Thanks.
You end with a phone that believes that it has never been used.
If your contacts are synced with Google (or with Backup Assistant), they will be returned when you sync again for the first time. Pictures and music are always stored on internal memory of the external SD card, and these are not wiped, so they will remain. I'm not sure what you mean by "other stuff" - if you mean the data from third party apps, or text messages, or call logs - yes, you "lose" those. You'll also likely need to reinstall third party apps.
To get back third party apps and data, after rooting the phone but before using safestrap to swap the safe and non-safe systems, you can install the application Titanium Backup Pro
(that's the market link) and (after starting it and allowing root access) do menu->batch->backup user apps and data. Then, after installing the custom ROM (and Google apps) and wiping data, after restarting and going through the setup, you can reinstall Titanium Backup Pro from the market and menu->batch->restore missing user apps and data. This will not get back system data like text messages and call logs, though there are ways to backup and restore them using standard market apps.