I built a media center computer that's running Vista Ultimate and running to my bigscreen via a DVI to HDMI cable and I'm very happy with it and it's been very stable. To date I haven't had any problems with hardware issues. Usually if you buy good/quality hardware there's ample support for it. I'm not running anything fancy - Asus board with Nvidia chipset and AMD X2 processor, an NVidia 6200tc (fanless to keep things quiet), and a 380w Antec PS that came with the media center case.

One thing I will say for Vista is if you have shitty hardware it will show it by running like ass. I went with a WD 400GB harddrive with 16mb cache. It's not Raptor speed, but it gets the job done. I left 1GB memory out to see how it would run until I put the full 2GB in and it made a HUGE difference. Memory and HD performance are crucial in Vista to make it run well. And if you are stuck using many of the onboard videocards that big-box companies have loved to use in the past because they were super cheap to build (read: Dell, e-Machine, etc) you might not be able to ever run the Aero features in Vista. I've seen a quite a few Dells that had no AGP or PCI-e 16X recently.

My guess is Microsoft put the performance indexing into Vista to show people if the computer they were getting was really any good. I can't imagine how poorly those $299 and $399 computers Dell was pumping out perform in the benchmarking.