From a web design perspective it really doesn't matter
what OS you use. I use XP and hand code with Notepad2
Depending on the project if you are using Flash on the
website you basically have M$ OS or MAC OS unless
you use emulators to get this program up and
running on other OS's.
Also developing for MAC IE5 should be taken out of the
equation in development for a website. MAC's not
supporting IE5 and has opted Safari as its pre-installed browser.
INTERNET EXPLORER FOR MAC NO LONGER AVAILABLE FOR DOWNLOAD
If your not dependent on using an application to create websites
then migrating to an OS like Linux shouldn't be a problem.
More important would be User Experience
, testing browsers on
multiple platforms for issues that may hinder the user experience