MythTV is great. I have mine set up with the Hauppauge PVR-350, and it works pretty damn good. The PVR-350 is great, because it is a hardware MPEG2 encoder and decoder, so it really takes a lot of load off of the CPU. The only thing I regret is that when I built it, I was going for size, so I used a
Mini-ITX motherboard, which is kinda slow, so changing channels is a little laggy for me. But I am eventually going to build another one with a decent motherboard and processor.
If you don't have much Linux knowledge, then you should probably go the Windows route. I have heard that
SageTV is pretty good. (They also have a Linux option now)