Microsoft has its own test, the Windows Vista Upgrade Advisor. It provides a fair amount of information on your PC and does even better with advice on which of your applications are likely to have problems with Vista (although some of the issues it was identifying last fall may have been cleared up as the backlog of drivers needing updates has been reduced). But the Upgrade Advisor runs only on PCs that are currently running XP.
That brings up another factor the tests don't cover. If you've got a PC that's pretty robust, fairly new, and running Windows 2000, you may be a better candidate for a Vista upgrade than an XP user -- but you'll pay a higher price not just in cash, but in effort. Vista upgraders can perform what Microsoft calls an upgrade in place only if their PCs are currently running some version of XP. If you're still running Windows 2000 you can save a few bucks by buying an upgrade version of Vista rather than the full package, but you'll have to do a so-called clean install -- move your files off the PC's drive, install Vista over Win 2000, and then move your files back and reinstall your applications.
The upgrade paradox is that the older your PC, the more likely you are to get real benefits from upgrading it, but the less likely it is to be economically worth upgrading. And that's affected not only by the age of the graphics card, the amount of system memory, or the speed of the processor, but the version of Windows you're currently running. And that's why, in the end, most PCs won't be upgraded, no matter what their test scores look like.