Do you want to erase this iphone & sync with this library?

Ok I have been searching for days about this now and I still cannot find a simple answer, and the reassurance I need.. I know there are a million posts about this, but they just don't answer it simply for me...

I am trying to sync my iphone 4 with my itunes because I can no longer update the music on my phone.

I get the message "Do you want to erase this iphone & sync with this itunes library."

If I click yes, to erase & sync, will I;
- Lose my contacts?
- Lose my photos?
- Lose my messages?

Or does this only erase & sync up music & apps?

I have transferred my apps to my itunes library, but the box to 'Sync Apps' is not ticked. Do I need to tick these boxes for apps, music, photos if I am going to erase & sync, or do I leave them unticked?

I have backed up my iphone with itunes.
I really do not want to lose my messages & contacts.

Any simple advice please?????

Ok good to know.

So if I right click on my iPhone in iTunes and click 'Back up' then click erase and sync, it'll wipe everything? Then do I go restore and it'll back it up to what I just saved?? :-/