I read about this somewhere, and apologies for not remembering where. But the 3G to 3GS upgrade was meant to focus more on the internal part. Particularly the CPU. It wasn't about a faster CPU or anything, but it was about the architecture of the CPU. iPhone 3GS uses a CPU architecture that is largely similar with the A4. I think the author of the article I read it called it an "A3." Again, apologies as I don't remember where I read this.
But anyway, that's the reason the 3GS can still handle iOS 5 and iOS 4 almost as well as iPhone 4 can, and can handle games incredibly well, when compared with the 3G.
You can look up the specifics of the CPU on wikipedia:
http://en.wikipedia.org/wiki/IPhone_3GS#Processor_and_memory
http://en.wikipedia.org/wiki/IPhone_4#Processor_and_memory
You'll see that they use the same base CPU architecture, the ARM Cortex A8.
