My 2c worth:
XP will do just fine until Microsoft stops releasing fixes for it - this is probably the OS to stick with for now if you've already bought and paid for it (though if you're stuck at SP1, I wonder!!!)
Vista - avoid unless you're buying a new PC with it preinstalled or really really need to play a game that requires DirectX 11.
64-bit Windows - carefully check your hardware and ensure there are (tested) drivers available.
Ubuntu makes a great secondary (or primary!) OS. For the usual basics of web browsing, email, office-type applications, graphics, etc, there are massive amounts of free software, all pre-packaged and just a couple of mouse clicks away.
There aren't usually any dual-boot issues, unless you install Ubuntu first and then Windows (where Windows will happily temporarily splatter any non-MS boot menu). Recent versions have full NTFS read/write support so you can get to your Windows files from Linux and will quite happily run quite a bit of Windows-based software via WINE.
Apple's Leopard will probably do well on the proprietary hardware that runs it, but will always be a niche market.
And as for AmigaOS 4... Cough.... Well, the three AmigaOne users and the four people with classic Amigas and PPC accelerators held together with sticky tape and string should run it and take over the world just fine! :-D
I have a mixture of hardware and OSes, and use Amigas for nostalgia, WinXP for stuff that requires the use of commercial software, Ubuntu for a web/file server and to do usual day to day tasks like web browsing / emailing / creating documents, etc and XUbuntu for the same on an elderly laptop that would otherwise run Windows like a dog.
Just my opinion!
- Ali
