The transition is going to be easier than you think. I was a Windows user last year (I still have to use the OS daily), and I was using Mac OS X like a veteran in a mere week.
In Windows, you generally have a window with a very complex, cluttered UI (this isn't meant in a derogatory way, as a lot of pro Mac apps are the same way, and it's simply the nature of the program). Most programs on the Mac, however, will have toolbars that give you shortcuts to often-used features. The other features that you don't use that often are up in that nicely-structured, omni-present menubar at the top of the screen.
Contrast this to Windows, which usually has everything placed in contextual menus, accessed by right-clicking. In that operating system, a lot of commands are hidden until you stumble upon them in the process of right-clicking. It's difficult for a Mac user to go to Windows, because of this, but Windows users shouldn't have that much trouble moving to the Mac OS, because everything is presented up-front in that menubar.
Additionally, this menubar is very consistent. Your Preferences window is ALWAYS in the "Application" menu (just like File, Edit, etc., there is a pull-down named after the application, which handles system-wide commands). You'll also always be able to quit the application from this menu.
In Windows, it's difficult, many times, to figure out whether or not a program is running, because some programs appear in the taskbar, others appear in the system tray, and others still, have no absolutely no
way to keep track of them. In Mac OS X, every program or application you're running will always appear in the dock, so you'll always be able to tell exactly what you have open, and you'll always be able to control your working environment. Additionally, each application is self-contained, and nothing is just "part of the operating system," as you have experienced on Windows. For example, you might ask, exactly what is the Display Properties control panel item, in Windows? Is it an application? Part of WinExplorer? No one knows, really. On the Mac, there is a single application, "System Preferences," that lets you configure aspects of your system.
Speaking of apps, being self-contained, they are literally a single file (actually, a "package," which, through secret methods, can be explored in itself). There is no need for a Start menu, because the apps are just one file and not hierarchies of DLL's and whatnot, as in Windows. You could make your own custom Start menu, if you wanted, by simply dragging the Applications folder to the Dock, and clicking on it, to get a "drop-up" list of any app you want. Installing apps are a breeze, in many cases: you'll just drag the single file, representing that application, to wherever you want to keep it, and then double-click to open it. Uninstalling is even simpler: just drag the application to the trash.
Drag and drop is big on the Mac. Want a program/application to utilize a file? Just drag it into that window or the icon in the dock. You can do this with practically anything, and the result is always logical. It's something that can't be explained very well, but once you try it, you'll understand it.
I hope this helps. It's really not as complex as I just made it out to be with those forty-three paragraphs, but it's going to be super-easy. Take it from a switcher.
Oh, and about the PowerBooks: buy one now. You have an opportunity to buy extremely fresh hardware (they've only been out a week or so), so just go with it. I have an 800MHz Ti, and it is a godsend of a computer. Now you can get a better model, cheaper.