Snapshots are also where Fusion and Parallels diverge in a significant way. Fusion's snapshot facility is fairly simple, allowing you to set a single snapshot, then return to or discard it. Parallels has a sophisticated snapshot manager that lets you keep multiple snapshots simultaneously. You can return to any of these former states and run them -- while keeping any changes you've made to the system in a new snapshot. Further, you can fork multiple changes from a single snapshot, resulting in a hierarchy of machine states that can be revisited at will.

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Taking a snapshot on Fusion happens so fast I wonder if anything's happened. Parallels can take 10 seconds or so to create a snapshot. Still, that's a small penalty to pay for the piece of mind that snapshots give.

Resource requirements
One of the more important questions for anyone considering virtualization is resource consumption. Both Parallels and Fusion allow you to easily constrain the resources that a given VM uses. My recommendation is to accept the recommended defaults unless you have a good reason not to. In any case, you can always change these settings later.

Running virtual machines isn't for skimpy hardware. The two resources that matter most are memory and disk space. There's no way to get around the fact that virtualization requires substantial amounts of both. I ran my tests of Parallels and Fusion on a MacBook Pro with 4GB of memory and 200GB of disk.

Each running guest takes a substantial amount of RAM, as much as several hundred megabytes. The amount of RAM in your machine will limit which other programs you can run alongside the virtual environment, what you can do on the guest, and how many guests you can run simultaneously.

Most users will have one or two guest images -- power users may have dozens. A virtual machine image consists of configuration information, a memory image, and a disk image. The disk image is the largest of these files, typically starting at a 2GB to 4GB for Windows and increasing as you install applications and add files. Both Fusion and Parallels support sparse disk images that use only as much physical space as necessary.

When you first start using virtual machines, it's interesting and helpful to keep an eye on OS X's Activity Monitor (Applications > Utilities > Activity Monitor) to monitor resource usage with different workloads. For example, I found that Parallels and Fusion both consumed 5 to 10 percent of the CPU when running Windows XP, even when the VM was idle. With Word loaded, you will see CPU usage run as high as 20 percent, even when you're not using the application.

Managing VM images
Moving a virtual machine image from one location on the disk to another is easy. Because machine images are just directories, you can move them to a different location on the same disk without breaking anything. Beyond that, however, doing things with machine images requires some management tools.