Microsoft now has its updated hardware requirements for Windows Vista, and as I figured, they aren't as bad as many were saying. Let's take a look...
Personally, I've been testing on a P4 3GHz with 1GB of RAM and a $99 video card I picked up for the nice graphic effects of Glass. Even the beta builds, which are always slower than release, have given decent performance - and that's with all my development tools.
Under the term "Vista Capable PC", which I take to mean it'll run like XP, then it wants a modern processor of at least 800MHz. That's easy - trying to find a computer in the last couple of years without that is the hard part. Then 512MB of RAM and DirectX 9 video. Well, the memory is pretty low - basic home computers today are 1GB. DirectX 9 video has been supported by on-board video for some time, again no problem.
What they call a "Vista Premium Ready PC" is a bit more. Again, not a problem for most computers bought in the last year, but it's a bit more tricky to hit all the requirements...
1 GB RAM, as a minimum requirement, seems fine. As I mentioned, that's what I've got and while more memory would always help, it isn't making it unusable. This may be a problem for older computers or laptops.
The 128 MB of graphic memory I think is the bigger issue. Now I, as a hardcore gamer, don't have a problem with this. But anyone with a laptop is unlikely to be able to run the 3D Glass effects (which is all that this requirement is all about). Desktops can pick up a video card that provides this for around $100 if they don't already have one. Of course, the point here is that you don't get Glass, but you still get Aero, the security improvements, search, etc.
DVD-ROM drive. Again, something that has been pretty common on desktops, and even laptops. DVD isn't fundamental to running Vista, but I believe that's the only medium the installer is going to ship on. I think an external DVD-ROM might be a good upgrade for older computers.
So, end story - if you bought the computer in the last year or two, you likely have a "Premium Ready PC" not counting the 3D Glass effects. If you want to make sure to get the full effect, look into a machine with on-board video RAM.