The problems with Vista will not be only if you pay or not...you will pay for sure, and not just because you'll upgrade your O/S. Have a look here:
And you will get a "safe machine"...that will control everything, including you!
Perfect DRM and in the future, you will discover that you can be prevented from reading your own files (created by you, not just the ones you bought or copied), if you finnish a file related paid service or your software license expires! Enjoy!
The MAC will require that you buy a new machine and brand new software.
Linux distributions are free, you have a lot of open source software, but if your interests make you to need the very last games you will probably be "forced" to go along with Mr. Gates new wander...