[fungrified]

when you plug in your phone to itunes it automatically backs up everything.
you can do it manually by right clicking it in the devices list and click "backup now"
it will probably take a very long time :S

after you jailbreak it and plug it into itunes instead of clicking set up as new phone
click restore from backup

all of your appstore apps will be saved, and i beleive also your jailbroken apps too but dont hold me to that