DO NOT load iOS 4 on an iphone 3G
The 3G was always slow, but with iOS4 is is unbearably slow. It is like a brick. Do not update your operating system you will regret the decision immediately!! I think Apple developed this operating system as a strategy to get 3G owner to get buy a new iphone. Don’t fall for it. Most of the new features of iOS 4 don’t work on the 3G anyway, so you get almost no benefit in exchange for the slower performance.