What do you have now? Will you be upgrade-eligible on or around the time the new one is rumored to be released? If you're not in desperate need of a new smartphone right now, it might be a good idea to wait.
Otherwise, yes, IMO there's a huge difference in speed between the 3G and 3GS. My brother has the former and I have the latter (jailbroken) and I was astonished that the difference was that marked. The main place where the speed differential showed up when I used both was in trying to use Google Maps and drive (I was a passenger, not the driver), following their directions. There were times when I wasn't sure if the 3G would be able to give me the next step before we'd passed it, especially if a recalculation was necessary. With my 3GS, I never have that concern.