+1... the highest iOS version the 3G will allow is 4.2.1. However, as Unleashed mentioned, iOS 4.0+ ran horribly on the 3G. iOS 3.1.3 (the last iOS 3 version) is the best running version for the 3G.There is no way to install iOS 5 on the iPhone 3G and even if you could I would not advise it as it could barely handle iOS 4