I record in HD and find that each 1hr show takes up approx. 8.2GB.
I don't have an XBox 360, nor do I stream any content, but I have a couple of search results for the Xbox 360 on sites that have great resources:
Home Theater PC News is a great site that really helped me with my HTPC build.
AVS HTPC Forum has a ton as well.
Here's a new article from PC Magazine about the new
Dell XPS 410 that will allow you to use a cable card for recording HD off of your
cable's system feed . It's only going to be available to major manufacturers, not the DIY crowd for a while, but it's still a promising step in the right direction.
