Can I still upgrade my iPhone 3G to iOS 4.x?
The final iOS rev for iPhone 3G is 4.2.1, and you could always save a backup before upgrading and restor to the 3.X version if 4.2.1 was too much of a hog.

Hope this helps!

Edit: This thread says its not even worth it. 4.2.1 is crippling to the device.