I'm probably the only person here able to say that I succeeded in doing this. My parents use Kubuntu.
Despite what people say, I've found that they're more comfortable with it than they were in Windows, and they've never had any problems with it. Things haven't randomly broken for them like Windows.
The way I did it? I just waited for Windows to break. That was enough of an excuse to them to try something they knew I believed it. I setup everything for them before they ever touched it. I installed all the apps they needed, and they've never had any problems with any of it.
I tried to switch them a couple years ago, but there were a few things that just weren't ready. I think it was Kubuntu 7.10 that eventually kept them for good. (Took two tries to switch them - they hate Windows now and have a hard time using it
My advice is to make sure everything is ready for them before they ever see it. You may even want to put a picture of their dog as the wallpaper or something. Whatever you can do to make it personal for them. The little things like that bug a lot of people. If everything seems foreign to them, they probably won't enjoy the experience, and they won't even give it a chance.
I think you should switch them to Firefox first. If they aren't used to that yet, there's no way you'll get them away from Windows.
As for getting a Mac, I've found that of all the people I've switched OSs for, OS X was the hardest for people to become accustomed to. KDE is a familiar experience right out of the box. Even my mom didn't have to spend any time looking for stuff. She recognised the K menu as she would the Start button, and found all her favorite programs already there, even sorted by category and not by name (which made it easier for her, because even in Windows she had a hard time finding the program she was looking for because she couldn't remember the name of it).
My mom already loved gimp, firefox, and (then) gaim before she ever switched. I think that's one of the things that really helped. What's funny is that more of her programs work in Wine than they did in XP (she has a lot of old educational stuff written for Win3.1 and 95).