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Hmmmm, before you do I would just say that the Android Market and paid apps are tied to the google email you set the phone up with. If you change the email in the Talk account you will have to change the whole phone as I don't believe there is a way to sign in to Talk with multiple accounts.
Edit....
Just had another thought, have you tried any of the third party apps from the Android Market which allow chat... there are loads. They might allow you to sign in with a different google account.