There is now a beta version of the Windows Vista Upgrade Advisor available for download. This tool can be installed and run on a Windows XP computer to assess how well the computer will run Windows Vista. There are several categories of Windows Vista functionality that it will evaluate, and you can pick and choose the ones that are important to you.
One of those functional categories is Watch and Record TV, so you can use this tool to evaluate how well Windows Media Center PVR functionality will work on a system if you decide to upgrade it to Windows Vista. The Upgrade Advisor also has a link to more details about Windows Media Center features in Windows Vista so you can learn more about some of the specific features in Windows Media Center in the upcoming version.