If you compare the iPhone 3G’s internal hardware to the iPhone 4’s you will see that it has less than half the processing power and a quarter of the RAM, therefore there is a noticeable difference in terms of performance.
Many people noticed the difference when they installed iOS 4.0 / iOS 4.1 on their iPhone 3G, performance was extremely poor and most people decided to downgrade to version 3.x, whilst the iPhone 4 came out of the box with iOS 4, therefore ran it without an issue.
A video recently posted on RedmondPie shows iOS 4.2 running on the iPhone 3G, we have embedded it below and you can clearly see that performance is much, much better, there is still a little lag, however I think most iPhone 3G owners will be happy with the update.
To be honest with you I would install iOS 4.2 on my iPhone 3G if I had one, will you?
Also See: iOS 9 beta 6 likely for testing partners