can i upgrade with odin and keep all my apps and contacts?
well for contacts you can use your gmail account.
Settings>accounts and sync>enable back ground data, enable autosync.
at the bottom select your Gmail account,>sync your Contacts.
then from your computer log onto gmail look in the contacts area and they should all be there
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for your apps you can use titanium backup to back up your apps, but when you switch to froyo you can only restore missing apps+data... not missing apps +all system data, if you do that you will have a force close problem.
you can also get an app brain account online, then download it to your phone and sync, this will record all your apps, then after they update you can sync with app brain again and all you have to do is reinstall them all.
using odin to froyo will leave you with an out of the box fresh froyo phone , so you will have to do all the setting up again