I second the Hauppage card + ATI combo that is a SWEET combo.
Instead of the Vista/media center options you may want to look into Linux's "MythTV". Heck their is even a Linux distro with MythTv all setup and ready to go for the beginner (KnoppMyth).
Here is some URLS if you are interested:
MythTV My Settop Box MythTV | MythTV Information and Resources Linux.com :: Three MythTV Linux distros compared A Beginning Look At MythTV
Good luck!
AkG