When I began speaking at CLE programs on law office technology issues in the mid-1990's, my standard advice was to stay with the operating system that came pre-installed on your PC. Each new OS had increasingly demanding hardware requirements, and the upgrade process did not often go smoothly.
In 2007, one would hope that this situation had improved. It has not. Many current PC's that are perfectly serviceable running Windows XP will not be adequate for Windows Vista (or at least won't run the new graphical interface called "Aero" that is one of the primary reasons to upgrade to Vista). But hardware suitability is only part of the problem.
Instead of two (or three if you count Media Center Edition, which wasn't available as an upgrade anyway) versions of Windows XP, computer buyers and OS upgraders must now select from four versions of Windows Vista. And the upgrade path from XP is anything but clear or logical.
First, XP Professional, which is what you should be using in your offices (and probably home too, but that is more debatable), can be upgraded only to Vista Business (about $200 for the upgrade), which lacks the neat Media Center features you may want, or the more expensive Ultimate Edition (about $270 for the upgrade).
XP Home can be upgraded only to Vista Home Basic ($100 upgrade) or Home Premium ($160 upgrade).
To make matter worse, upgraders have always known that a "clean" install of a new operating system was the best way to go. That involved backing up your data, wiping out your old OS, reformatting your hard drive, and starting from scratch. You could do that with upgrade versions of Windows before Vista. You just needed the install CD for the old OS to prove you were entitled to the upgrade price. There was a point in the clean install process where you would be asked to insert your old CD for verification. Simple!
Not so simple with Vista. The upgrade version works only if you are installing over an existing version of Windows (and then only if it is the version Microsoft has deemed is part of the proper upgrade path as described above). You can either pay a lot more money for the "full" rather than "upgrade" version of Vista, or you can go through the redundant step of reformatting your hard drive (after backing up your data), reinstalling either your old OS or install the demo version of Vista (from your upgrade CD, just don't activate it), then do the upgrade install of Vista. Yes, this is incredibly stupid.
The bottom-line is that if your PC is running fine with XP, stick with it. Save Vista for when you buy a new PC with Vista preloaded. And wait to do that at least until Vista Service Pack 1 is released and you receive confirmation from all of your software vendors that the programs you depend upon will run properly on Vista. That could be 6 months to a year.