A quick comparison between the 2G and 3G iPhones specifications

Dimensions
2G: 115mm high, is 61mm wide and 11.6mm thick, weighs 135g.
3G: 115.5mm high, 62.1mm wide and 12.3mm thick, weighs 133g.

Display
2G: 480 x 320 3.5″ multi-touch display.
3G: 480 x 320 3.5″ multi-touch display.

Storage
2G: 8GB and 16GB (4G was discontinued later)
3G: 8GB and 16GB, with the latter available in both black and white.

Camera
2G: 2.0-megapixel camera.
3G: 2.0-megapixel camera, with geo-tagging and third-party application integration.

Connectivity
2G: Wi-Fi, Bluetooth.
3G: Wi-Fi, Bluetooth, 3G internet connectivity.

GPS
2G: None.
3G: Yes, assisted GPS.

Battery Life
2G: eight hours of talk time, up to 250 hours of standby, six hours of internet use, seven hours of video playback, and up to 24 hours of audio playback.
3G: up to five hours of talk time using 3G, or 10 hours using 2G, standby is 300 hours, five hours of 3G internet use and six hours of Wi-Fi internet use, seven hours of video playback and up to 24 hours of audio playback.

*hint: always double check the list from the Apple website above against the iPhone 3G Part Numbers (this could be found outside the box or via Settings -> General -> About -> Model) to determine the origin of your iPhone 3G, as there is currently a scam going around the market.*