trying to update my Iphone OS but it says: you have purchases have not been transfered to your itunes library . dont want to lose anything 've purchased dont want to cross reference every app and song to see if its in itunes or not. help

II have an iPhone 4. I want to update my iPhone operating system but when I do, it says I have purchases that haven't been transfered to my iTunes library. I sync my phone everytime I plug it in (automatically), so I'm not sure why I'm getting the message. I have updated opertating systems before and lost everything, photos, contacts, music, aps, etc so I've been hesitant to do it this time, but my phone is just too slow and aps keep crashing etc.

Short of going through and manually verifying every app and song ive purchased, how can I figure out what is missing from my library, and begin this update?

Thx

Jon