If you're looking for a media center and you're comfortable messing around inside a computer case, you'll want to strongly consider building your own system. Oreilly has a really nice tutorial that walks you through building a media center running MythTV, the extremely popular GPL home media suite for Linux.

The tutorial is a multi-part article and starts with the hardware and most importantly the TV capture card:

This one is easy. The HD-3000 from pcHDTV is one of only two capture cards for Linux, and the direct descendant of what was once the only card available. I purchased multiple cards so that I could build a multiple-tuner system. Furthermore, Jack Kelliher, the founder and CEO of the company, is deeply committed to open source software's role in television.

Overall, this is a excellent article to understand what's involved in building your own media center box.

Read Building My MythTV Box, Part 1: Hardware