some basic questions about rooting

Hi. all

I am using desire (Froyo) very happily except interal storage usage.

So, I have some interests and questions about rooting.

1. In Froyo (non-rooting) , some parts of app is stored in sdcard but not all.
    Will rooting make all parts of app move to sdcard ?
    In terms of internal storage usage, will rooting give me some benefit over froyo ?

2. If I root my phone, shoud I flash my phone with another customized ROM ?

3. If I root my phone, will my setting and apps and data disappear ?
    is there any backup methods ?  is My backup pro enough to restore my setting ?

4. If I fail to root my phone, can I recover to my original state ?

Some advices make me very happy and thank you in advance.