The system I run Myth on is not a powerhouse but it's not a turtle either. With an AMD 2000+ and 768 MB of RAM, it's probably slower than your desktop but it suffices for video recording and playback quite well.

Unfortunately after reinstalling Myth I had some audio sync issues and staccato video. I finally realized what the problem was.I had compiled MythTV with every video playback method known to man, thinking this would give me the most options. The downside is that certain things like OpenGL sync are always on when compiled in.

I ended up recompiling MythTV and only enabled XvMC playback support while disabling all other video modes. Playback is fast and smooth now, and the quality is high. I'm recording at 720x480 at a minimum of 4800 Kbps. It's generating around 1.5 GB of data for every hour recorded.