syncing feeling

my old iphone 3gs is running on ios4.2, and needs updating, but its linked via itunes to my old laptop, which is virtually useless. it takes forever to get started and fails to complete most commands. I have managed to connect to it but it failed to sync to my phone properly, so i couldnt transfer all my more recent purchases. I was advised to put itunes on my new laptop, download all my purchases, and sync to the new laptop. Doing this however results in a warning that all my data will be deleted and to save it i must use my old computer to back up my data. I dont want to lose anything, but dont know what to do next. i basically have 2 laptops with my itunes account on, neither of which will let me update my software without deleting data in some way. I am utterly confused as to what to do about this, so any and all help is appreciated, thanks 🙂