I built something like this about two years ago. It was fun, but ended up costing at least twice as much as a TiVo. If you still want to push ahead, I can testify that using Hauppauge PVR-250 capture cards and SageTV software works pretty well.

Another suggestion is that if you do this, install Logmein on the computer. This will let you control the computer remotely, so if you find yourself out of town you can still make or change recordings.