Tom,
Granted, some things will be different, but when you get right down to it, pretty much all windowing systems and/or graphical programs behave similarly in the grand scheme of things. Sadly, much of current Linux (and other free unix-like systems) graphical interface is becoming more and more MSWindows-like. You really should not have as much of a problem learning "Linux" for day-to-day stuff as you would have learning the Mac way of doing things. And if/when you choose to venture beyond the graphical confines to the very powerful underpinnings, you'll have a much easier time with Linux than either Windows (which doesn't come with any decent utilities to speak of) or Mac (which, it would seem, tends to discourage such work as well, but does include most of the good stuff out of the box).
Order the free CD from Ubuntu. The package comes with two discs: one for installing a system "natively" and one for running a live-off-the-CD system. Knoppix is also quite good for that and can be freely downloaded (bootable CD-ROM image) through many places found at
http://www.knoppix.org/
Knoppix is
KDE (K Desktop Environment) based and Ubuntu is
GNOME based (there is a KDE version of Ubuntu called Kubuntu, but is only downloadable and not freely shipped).
I don't use the GNOME environment as a whole (but do use several of the programs individually) but understand it is the more MSWindows-like of the two.
Get a bootable live Linux CD and try it. No harm done to the computer whatsoever; it merely runs a tad slower than it would if installed natively (which is quite a bit faster than MSWindows on the same hardware, BTW).