Yes the new Phone would restore your 3G back up, and you would also just put in your existing sim. Don't think it would need activated as your sim is already in service.
I had my 3G replaced last month for a new one and after restoring my back up I don't remember having to activate it again.
Carphone warehouse would give you £170 for the 8GB 3G and £180 or the 16GB 3G. They do trade in on their site now.
Also play.com have an unlocked 32GB 3G s for pre order......its only £999.99!!!!!!
http://www.play.com/Mobiles/Mobile/4.../Product.html#