Thanks for the comments and responses!
Without getting into a Mac vs PC vs Linux vs amiga debate, I'd have to say that WinXP Pro is the most stable OS I've ever used.
I do all sorts of stuff on my two PCs (IBM Thinkpad R51 & HP D530S) and I've yet to come close to foobarring XP, that's despite me being a bit of a registry hacker!
I've tried Linux and it's as bloated as XP, but even more obscure and difficult to work with behind the scenes.
When I was a daily Amiga user (now I'm WinUAE only) I tried out Mac OS (8.??) on Shapeshifter and was dismayed at the way it was so difficult to actually get to grips with the OS on a low-level.
The G3 iMac I'm looking at is only going to cost me around $AUD 50, which is a bargain. I'm planning on having a bit of a tinker with it, then pass it onto my parents as their first computer - they'll just be using it for email and web-browsing. Since the Mac is supposed to be a bit less virus and spyware prone than Windows, I thought it would be a safer bet for them.
Cheers,
Mike.