Don't buy a Mac, its way to expensive, and not worth it. Your paying extra for the brand, that's it.
Wait for Windows 7, as is pretty bug free. Been running the release candidate for months now and its the most stable OS I have used in a long time.
Being able to test drive an OS, and get a feel for it would be a great idea though.
With Linux, you get the Live CD's, where you can run the OS on a CD to see if you like it, without touching your hard drive or changing anything on your computer.
With Windows, at least for now, you could get the release candidate of Windows 7, and give it a try. Not ideal, as you have to have a PC spare to try it.
I do wonder if VM is a option though? I know from the VMware site you can download images of pre-setup environments for example.
Perhaps you could get a virtual image of Windows 7, and a virtual image of Mac OS, and run them through VM Player on your current machine to see how you like both the coming operating systems.
Not ideal, but as there are no Live CD's for Windows, or Mac OS its the only other way I can think of, for test driving a OS without installing it.