The newer generation of Apple's operating systems come with a "windows" emulation mode. You change a few settings, restart the computer and presto! you're all set to run windows software.
But, obviously, support for all windows software will be scratchy as is the case with all emulation software. This is especially true of software coded by individual programmers and hobbyists.
Incidentally though, id software releases almost all its games in MAC versions too, so atleast as far as doom 3 is concerned, you shouldn't have many problems playing it on the OS X.
Also, now that it is running on an intel box, windows software can be ported to OS X with much greater ease than earlier, when MAC was using a G processor.