I just upgraded my Powerbook from 768 to 1.25 GB.
What I notice, things do feel a little snappier. The biggest difference is when I'm doing something processor intensive, believe it or not. For example, when I'm encoding in handbrake, or I am making a DVD in Toast. Those aren't applications that use a particularly high amount of memory. However, previously, my system would really lag, when I tried to do other things, while doing those intensive tasks. Now, I can do whatever I want, and the speed is there.
I'm happy I upgraded.