Do you live or travel in areas with 3G? This will make a huge difference if you like to use the wireless data aspects of the iPhone. Surfing real websites at Edge speeds; not so great. Perhaps you spend most of your time in hotspots, in which case the 3G upgrade will have less benefit to you.
Do you value GPS?
Do you like the updated design?
Improved audio (not sure exactly how this will shake out, but Jobs made claim to huge improvements)... is that appealing?
I guess, depending on your answers to the above... you can decide. Either way you will get 2.0, app store, and the option for MobileMe.
To me, this is a significant improvement on an already awesome phone. The lack of reasonable wireless data speeds was a big deal to me.