For the homescreen, I use Nova Launcher. It can back up the way it's set up - which apps and widgets, where, on which screens.
As far as the rest of it, you can back up your apps and data with Helium. You can back up your media (videos, pictures, music) by just copying them to your computer. You can back up your texts with SMS Backup+. Your contacts should already be Account contacts (not Phone contacts), so that backs them up to Google.
Then (if you're really unlucky, flashing a new ROM will kill your computer) back all the backups to a cloud account. Old computer axiom - any file not backed up to at least 2 separate devices is a file you don't need. (The odds of both your computer and Dropbox failing [and Dropbox not being backed up] at the same time are more than just astronomical.)
The first thing you should decide, though, is why you want to flash that ROM? Doing so voids the warranty on your phone. If it's just "to have a ROM", you probably won't be very satisfied. If the particular ROM has things the stock ROM doesn't, and doesn't leave out anything in the stock ROM that you need, that's a good reason. (By that last, I mean that if the 3rd party ROM doesn't do Bluetooth, but you're never going to use Bluetooth, that's fine. But if you use your phone to navigate, and the new ROM doesn't do GPS, it's not a good idea to flash it.)
Many things newcomers think they need a new ROM to do can be done much easier (and with less risk, both to the phone and to the warranty) by installing apps. You don't like the way the homescreens work on the G2? Try a different launcher or 5. Don't like the email app? K9 may be more to your liking. You don't need a new ROM to install wallpaper, or change icons. Decide what it is you want to accomplish first, and figure out the least invasive way of doing it. If that happens to be flashing a new ROM, and you're comfortable without a warranty, go for it.