Not able to add any app on my iPhone 3g

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Recently i upgraded my iphone 3g to ios 4.2.1 and jailbreaked it.When i sync with my itune. I get an error saying "Some of the Items in the itunes library were not copied onto iphone since the software is too old". When i check for software updates it says latest version is updated on the phone. Then same error occurs. Please help.

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