what's the oldest version of itunes I can use with the touch
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7.6
according to http://www.apple.com/ipodtouch/specs.html
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Essentially correct.

However, more specifically, you need iTunes 7.4 for the original iPod touch firmware (v1.1.1/v1.1.2), and 7.6 if you're using firmware v1.1.3 or later. So if you're using an older iPod touch and you haven't upgraded the firmware yet, you can get away with a slightly older version of iTunes.

New iPod touch units are going to come pre-installed with at least v1.1.3 for the most part, so iTunes 7.6 is a minimum requirement for a new iPod touch.
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