iPhone and iTunes Synching

Hello,

I'm just a week old Mac user, so please help me out with these silly and easy absurd questions, (imo). First of, my iPhone is synched to my old Windows-based laptop. When I plugged it into my Mac while using iTunes, I received a message that I have to erase all of the contents of my phone because it is synched with another computer. It's actually fine with me. The problem is the songs stored in my phone. Is there a way to get the iTunes library/settings specifically all of the music so that the songs will not be lost after erasing my phone? Also, I know whenever you create a backup of an Apple device like for example, an iPhone, all applications are copied right? Everything will be stored in iTunes so that when you restore the backup everything will be back and organized again specially the folders of my apps. Well, I just find those apps stored in my Mac since I won't be able to use them, right? If only I can backup everything, yet the apps will not eat my disk space or just to simply delete them after synching.

Another thing, I also have an iPad. Is there going to be an issue when I plug it to my Mac, through iTunes that was used to sync my iPhone? And I have lots of pictures in my iPhone. How can I back them up? Aside from using iPhotos, what other applications can I use? I just wanna backup the Pictures and Videos to my Mac but not restore them. iTunes restores everything right?

Thanks.