Apple supported the 3G from iOS 2.0 to 4.0. 3GS from iOS 3 through iOS 5. From the beta, it seems like the iPhone 4 was the target machine for iOS 5, so it looks like it should last until iOS 6 or even longer. The original iPhone and iPhone 3G had really poor processors, but the processor in iPhone 4 (and RAM) looks like it is better built for the future.
A lot of the camera apps just let you do more stuff to it, like add effects or whatnot. I have Camera+ but really just use the normal camera app on the iPhone. Looks like Camera+ has an increased digital zoom that looks better (but I say stay away from digital zoom altogether), let's you crop photos, an image stabilizer, and a lot of effects and borders you can add to your photos.
The Verizon iPhone 4 uses a different antenna. I think it still technically has that problem, but the "weak spot" is no longer in that same spot, so it has a less of a chance happening, and I've also heard that Verizon's signal is strong enough to for it to rarely happen.