Actually, the MythTV FAQs still only recommend single core CPUs. I recommend a dual core CPU simply because the more CPU power you have, the better. It's not actually because the cap card does the encoding, it's because once you build one, you realize how versatile it is. I'm in the process of putting my entire DVD collection on mine. The faster the CPU, the quicker I can encode the DVDs to smaller h.264 videos. I get nearly the same quality, regular audio and commentary, and about 1/8th the file size.

Of course, the faster your CPU, the less you'll need a capture card that can do encoding. If the CPU is fast enough, a simple USB encoder would work just fine.