Hello Simon,
I have a ThinkPad 600 compliments of a generous group member. Its a 300mhz maxxed with 296MB of RAM.
I'd suggest you try Puppy Linux with those specs, in order to get the most out of the machine - Firefox and Openoffice are both available. You wouldn't be able to run Itunes without wine or a vm software, but I don't know of any Linux distro that does.
As for other operating systems, XP may well be happy running on that machine, but expect it to become unusable fast with most current antivirus software (not to mention Itunes).
I know I have asked this before. Ubuntu and XP are ok, but are quite sluggish at times > and it is frustrating
Again, it really might be worth your while starting with something fast and minimal, and seeing where you can progress to, rather than installing and running everything including the kitchen sink, then trying to pare it down.
I think in terms of the fear of tinkering you mention, I think that while it might *seem* easier to get results starting out with Ubuntu 10.04, you could end up spending many hours on it and *still* have a laggier system than you're looking for.
Good luck.