Here's a really interesting video found on YouTube which shows speed comparisons of different iPhone's which include 2G,3G,3GS and iPhone 4G.
The video shows all the iPhone head to head running different applications which include Plants and Zombies game, Google Earth and Net Browsers. The iPhone 4 takes the lead in loading all the apps first with the most fastest browsing speeds while 3GS is just a bit slower to it.
This and the 3G and 2G are drastically slower than the other two variants which take a loading time of 25 Seconds as compared to the 8.5 Sec loading time of the iPhone 4 in Plants and Zombies. Same is with Google Earth and the browsing speed is also slower on the other iPhone's as compared to the iPhone4.
The browsing speeds in Safari are also much fast in the iPhone 4 which takes the lead and 3GS is pretty same. While 2G and 3G are left behind. You can check out the video below for yourself: