My personal thought is if your looking for something to switch all your sites to, and your considering MacOS then at the very least you should be considering MacOS X Server.
OS X Server has been out for awhile, and is rock solid from what I've seen. There are many strong arguments to be made for using Server versions of software, especially if your running multiple sites, etc.
OS X, the consumer version, will run Apache, and other web masters are switching (primarily those who were using WebStar, or other Mac based web servers), because comparitively OS X is a far better web environment than OS 9.
But if your sites are currently hosted on Unix or Linux, then the real consideration should be OS X Server on a G4 box, if you want the same power.
Alnisa