most of the improvements (speed, new apps, fixing bugs, etc.) are just software changes, so your old iPhone 3G is basically the same as the 3gs with the free software upgrade, the only real difference being that the new iPhone 3GS has a built in video cam function.
so, final answer, it is worth it if you do not have an iphone already (although the at&t service is a bit pricey)
if you have an iphone already, just get the upgrade. (and if you really need a pocket cam/video recorder, just buy a flip cam for like $99)
IMPROVEMENT: if you want to record video on your original iPhone or iPhone 3G, there's an app for that! (GET IT IN CYDIA)
iPhone 3G S = iPhone 3G + Extra's
Both the iPhone 3G and the iPhone 3G S support MMS with the 3.0 update.
iphone 3g s
No, the only phone with higher quality is the iPhone 3G[S], and the new iPhone that isn't out for you yet.
You can get a load of accessories for the iPhone 3G S, such as headsets, and of course car chargers.
Yes
1000$
A regular iPhone 3G (not the 3G S) costs about $99, give or take some depending on where you buy it (not including tax).
The app that you play a song to and it recognizes the song, only available for the iPhone, iPhone 3G, or iPhone 3G S, is called Shazam.
Selena Gomez Has The iPhone 3G [s] =]
It depends on what iPod touch it is but it's not really worth the risk.
The difference you will see between the iphone 3G/3G S and the iPhone 4 is quite noticable. Especially the design. AS for the iphone 3G,there is no recently used apps bar (multi-tasking), you can't have a background wallpaper on the homescreen. As for the 3G S, you have both of these features. However, the iphone 4 has the Retina display, which the previous models do not. The iphone 4 has facetime (video calling), which the previous model does not. The iphone 4 can do HD video recording, has 5.0 MP with flash. I believe the 3G does 2.0? and the 3G S has 3.0 MP to it. No flash on previous models. Really, you can just go to the iphone section and i'm sure you can compare between the iphone 4 and the 3G S. In US, they do still sell the 3G S model but in 8 GB of storage, each country could be different where they could still sell the 16 GB model or the 32 GB, but very unlikely, unless they have few in stock. iphone 4 comes with either 16 GB or a 32 GB.