I really have to disagree. As soon as you really start working with Mac OS X 10.1, you start having many applications open. *Because* the operating system multitasks well and organizes its memory well, there is no need to quit a Carbon or Cocoa application like there was in OS 9. It's much faster to just keep it in the background and let OS X handle it than to quit the app and later start it again when it's needed. Let's make an example...
I'm a webdesigner. And I work on OS X as my primary operating system since 10.0.3. I always have Internet Explorer and OmniWeb open. I never restart them (unless they crash, of course, which they rarely do). Then I have Photoshop (Classic) opened all the time, too, because I often switch back to graphical work when I've done some coding in Terminal or BBEdit. And of course I have Stickies open for my notes, I have Mail open all the time, I have Transmit (FTP) open and I have Fire open. All of them. All the time. Now if you have 128 or 256 megs of RAM, the system (10.0) starts to show you the rainbow cursor quite more often than normal. 10.1 is a bit better here, but not really. As soon as you hit the real memory line, OS X starts to stutter. I just upgraded my TiBook from 384 to 512 (replacing a 128 with another 256 brick), and I can work faster now. OS X doesn't need more RAM, all of your applications will, though. This will get better again when I don't need Classic any more, i.e. when Photoshop is carbonated.
Plainly put: Buy as much RAM as you can put into your machine. It's always gonna be a speed bump. Be it OS 9 or 10.