Why have you forsaken me?

My MythTV box has been chugging along nicely, playing DVDs and recording two shows off of a 2-tuner standard definition hardware MPEG encoder card (doing all at once). Just before Thanksgiving, I purchased a projector that can do 720p and had been enjoying the finer detail of my DVDs at full resolution. However, standard definition material really looks like crap when displayed on a 69" screen. Enter: High Definition. I bought a HDTV tuner card, thinking that I had prepared myself for this moment. According to the MythTV documentation, the minimum recommended processor speed for recording and playing off of one HD Tuner card is about 2.4GHz. So I built my box with an Athlon XP 2800+ (2083MHz clock speed) thinking it would be up to the task. Think again...