Mac-users, especially of the older power PC systems (G3, G4 and G5), who need to use Windows or DOS-based applications need to use Virtualization software. On Power PC Virtual PC seems to be the best choice in my own experience, although Qemu and Virtual Box are also quite usable. The owners of the newer Intel machines can use bootcamp that comes with Leopard to run Windows or another x86 OS natively. But even the new Intel Mac users might not want to reboot their machines every time they want to use Windows applications and two popular solutions catering for the Virtualization of X86 pc's running Windows or another x86 OS whilst at the same time running OSX are VMWare and Parallels.

Mactech made an extensive test, benchmark and comparison of VMWare Fusion, Bootcamp and Parallels 3.0. And extensive it was, in total the test-team performed more than 2500 benchmarks.
A reference Windows machine used in the tests was a Fujitsu Lifebook A6025 running Windows XP SP2, the other machines used were three poplar Macintosh systems: two Mac Books and a Mac Pro. All machines were tested using XP SP2 and Vista whilst running popular Windows applications.

Some of the conclusions of the test-team don't surprise me. For example the fact that Windows XP runs more smooth on virtual machines than Vista does. Vista performs up to 30% slower on the same machines. Hey on my old P4 3.4Ghz Vista ran so bad that I reverted back to Windows XP Pro SP2 in a heartbeat.

VMWare Seems to be the best choice when you do want to run Vista. VMWare Fusion offers a completely separated sandbox whilst Parallel offers a more integrated experience. Funny because the names of the products made me think the reverse was the case, but what the hell do I know - I am Dutch and not a native speaker of this lingo.

If you want to run Dos or Windows 3.11 applications you might want to try out Dosbox, especially on Intel Mac it just flies and version 0.72 has become very compatible with lots of games. I even managed to get Windows 3.11 and Gem/3 up and running.

The Mactech article