First of all thanks a lot to Octavean for this GREAT post.
Here's a step-by-step tutorial that allows you to set up your Theater 650 to receive both analog and digital channels under Windows Media Center on Vista. Follow the instructions carefully and you will be amazed. I tried here and it works like a charm!
P.S. I only tried on Vista 32bit, not sure if this also works on Vista 64bit.
*ATTENTION* The tutorial tells you to download and install Windows MCE Drivers on Vista but I got it working with the official Vista 32bit drivers.