Hey, my name's Cody, and I'm working with the Windows Media Outreach Team to help answer any questions users might have about Windows Media.
The other folks were right: in Windows Vista, Windows Media Center is no longer a separate edition of the OS, but is integrated within the Windows Vista Home Premium and Windows Vista Ultimate editions.The main bonus that goes with having a Media Center PC is that it makes connecting with your other media devices (TV, etc.) a lot easier, basically extending your PC 's functionality around your home.
Something to keep in mind is that if you'd prefer to not have to plug your computer directly into your TV to connect it with Windows Media Center (here's a link that shows the hardware you'd need to plug directly in: http://www.microsoft.com/windows/products/winfamily/mediacenter/getstarted/wmc-on-tv.mspx
), you'll need to get what are called Windows Media Center "Extenders." Extenders are pieces of hardware that'll let you connect your PC to your TV no matter where it is located in your home. Here's a great overview of the extender technology: http://www.extenderforwindowsmediacenter.com/overview.html
(side note- if you've got an Xbox 360, then you already own an extender).
If you'd like a bit more info, I recommend checking out the Media Center homepage at http://www.microsoft.com/windows/products/winfamily/mediacenter/default.mspx
I know I bombarded you with links, but I think they are all pretty useful. I hope they're helpful!
Windows Media Outreach Team