With Vista the biggest requirement will be the graphics card, though what card you need depends on two things:
1.) What you plan on doing with it. (In this case watching Hi-Def video.)
2.) The size and number of the intended monitors.
In short, the bigger the monitor and the greater the number of monitors the more powerful card you will need. In terms of dedicated graphical memory:
64MB: 1,310,720 pixels or less
128MB: 2,304,000 pixels or less
256MB: 2,304,000 pixels or more
Also, you will need to have at least 1.6GB/s of graphical memory bandwidth.
If you want to gauge your current PC's performance you can run the Windows Vista Upgrade Advisor.
The second biggest issue is RAM. 512M is the bare minimum recommended, while 1GB is the minimum for to meet Microsoft's Premium rating. Considering your intended use, I'd suggest no less than 1.5GB.
Processing power will come more into play based on your tasks, not the OS itself. (Vista ran run reasonably well on a 1GHz single-core processor.) Right now the best bang for your buck is the Intel Core 2 Duo processor line...even the lower-end models best AMD's current cream-of-the-crop.
Now, you have a choice between getting the computer now or after Vista is released. Right now not all hardware (speaking of graphics cards and TV tuners) can pick up right where they left off when going from XP to Vista, so that's a mild concern. (Look for a Vista ready sticker whenever possible on those items.) Also, AMD is planning on releasing its next line of processors and Intel is preparing their quad-core processors, so there will be more options and likely lower prices after Vista's launch. Thus, you may want to consider holding off for a couple months and then clean up when the timing is right.
Hope this helps,
I just bought a HD wide screen TV. I am also buying a new PC for the same room. I had the bright idea to use Media Center as a PVR for the new TV (I've got Tivo on other sets and love it). In talking things over I understand that Window's Vista Media Center will be supporting HD. If that is so, are there some basic recommendations on what type of PC configuration to buy today in anticipation of Vista's arrival next year? Most PC's come with 250+ Gig of hard drive memory so that's pretty good. I'm thinking most specifically about recommended processors and graphics cards. I am not a gamer. Any recommendations?