Hi! I am an iPhone 3G owner and mine is activated on AT&T, still works after six years. No, the 3G can't go past iOS 4.2.1, that's the newest it can get. iOS 4 misses out on iMessage and iCloud. There also is no 4G HSPA+ or LTE.
It's still a good phone for me though, and it works well for what I need it for as a teenager, calling, texting, internet, and music.
iPhone 3G, iOS 4.2.1