I was really surprised at how much of a bump I got from a 1.8ghz Core 2 Duo with 2GB of memory to a 2.53ghz i5 with 4GB. My poor Macbook used to choke if I was running more than a handful of programs, or if I was running Flash with anything. Now I can run a bunch of fairly processor-intensive programs (video, VOIP, Flash games, browser with 2 dozen tabs, etc) simultaneously and it rarely chokes.
Since I got such a big productivity gain from this change, I'm looking for other things I can do. I'll be due for an upgrade in another 4-6 months and I think I'll get an SSD.
What's so bizarre with this approach is that 2 years ago the specs on this were near 'top of line', and the software hasn't changed all that much over 2-3 years. I'm not saying it beach balls more now than 2 years ago - it's always been the same. I may do one more mac laptop - a higher end one next time - but I suspect my workstyle - doing multiple things all the time - is always going to end with beach balls, regardless of the mac. :/
I'll bet you'll find whatever versions of Flash you've had have steadily required more and more CPU. I read the crash dumps when my Safari becomes unresponsive and I have to force quit it, and it's always Flash.
I experienced something similar when I upgraded my semi-ancicent MacBook up to 4gb of RAM and a faster hard drive, which had amazing (positive) effects on its performance. I suspect that a lot of OSX's performance problems result from slow IO or limited RAM (leading to paging, and slow I/O again, etc.).
Since I got such a big productivity gain from this change, I'm looking for other things I can do. I'll be due for an upgrade in another 4-6 months and I think I'll get an SSD.