I'm always keen to know that the hardware I've got running at home is ticking along nicely. I do regular tune-ups of my PCs and basic error checking to at least give me a clue if there's trouble ahead.
Vista, with it's higher resource requirements than WinXP, is a good time to go through and do a thorough hardware inspection to make sure you don't upgrade and then run into problems.
That can be as simple as running the Vista upgrade wizard or it can be as hands-on and in-depth as opening machines up, blowing dust clear, re-seating the CPU, checking the bearings on the fans, checking the routings on internal cables etc.
It's harder to test some of the peripherals on your system though. For instance the router. Will it work with Vista? Will there be trouble? Does it support feature XYZ that Vista can use to improve reliability, throughput or security?
Sure, you can wade through the manufacturers support site and hope they have some decent (and intelligible) documentation (scattered with marketing double-speak of course) - and while you're there don't forget to check you have the latest firmware for your device.
Much easier to run the Internet Connectivity Evaluation Tool and make sure that it actually does what it says on the box.
This procedure performs a number of basic tests on your router to see what capabilities are supported. Some functions (ironically) can only be tested from Vista, but it does give you a good outline and idea of what's working and what's not.