I was in the same boat as you a couple days ago. I was running 256 ram, and every time i loaded dashboard, it would lag for about 5 seconds before it would pop up. I ordered a gig stick of ram, and now dashboard never takes any time to load, except for the first time it loads when you start the computer up... You're gonna notice a huge difference in performance and not just in dashboard. The whole GUI runs alot quicker for me now. If someone told me when i bought my mac that adding ram would make that big of a difference i would have upgraded right then. But then again, i really didnt notice the memory problem as much until i installed tiger.
it was a HUGE performance boost for me. If i only had 3 apps open, and i opened a 4th, it would take quite a bit of time... now, i can start my computer, open ever app in my dock and not bog down at all. You're gonna be shocked when you use it for the first time.