In "Which PC Is A Vista PC?" I write about the pitfalls of picking PC hardware that will run all the features of Vista you expect. For new-out-of-the-box PCs that shouldn't be as hard as it seems to be, as that article points out. But what about old PCs you might want to upgrade? Actually, that might be a little easier, because there are some tests you can run to help you decide whether an existing PC is up to running Vista.I have run several different "Is Your PC Ready For Vista?" tests over the past few months. I mentioned my favorite, the ATI Vista Readiness Advisor in a piece I wrote in November headlined Are You Ready For Vista Graphics? As you might expect, since ATI makes graphics cards, this test is particularly good at identifying the graphics capabilities of your PC. About the only thing it doesn't tell you about your PC's graphics card is its bus connector type -- AGP, PCI, or PCI Express -- which can be important because a graphics card is the hardware you are most likely to have to replace in order to upgrade an older PC to Vista.

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.