Like any other Unix-like OS, MAC OS X is RAM hungry. More RAM is the easiest and cheapest way to improve performance. Running Photoshop you will find a huge difference between 512MB and 1GB. Of course, it's also all in how you use your machine -- looking at my OS X Dock on my powerbook, I've got 15 open applications right now that I've been flipping between since my machine was rebooted a couple weeks ago.
-DJ
-DJ