After owning a 15" Apple PowerBook for over a year I decided that the installed 512 MB RAM was holding back the machine's capabilities when using memory instensive applications, like Illustrator. In particular, HP plotter drivers seemed to be capable of closing down all other apps while creating temporary plot files.
So I began researching the cost of RAM over a month ago. This week I was finally able to secure 2 GB of RAM, the maximum that can be inserted into the PowerBook's two dedicated slots. The price for this RAM varied enormously from brand to brand, from country to country. But Canada and Crucial won the day (at half the price of UK Apple!) and the memory is installed and the computer runs a lot faster and a lot cooler!
On this experience I would have to say that before considering a computer upgrade, first consider adding more RAM.
Also, for Mac users, consider downloading the free Menumeters application. It will help you to understand what is going on with the CPU, hard drive, network etc. in real time. Invaluable.