I'm taking another stab at setting up MythTV. After setting up a laptop using the openchrome drivers, I remembered how much fun working with graphics can be. I found the via unichrome system to be fairly competent, so I'm using a VIA EN-12000EG mainboard with a Hauppauge WinTV-PVR-250. From what I've read online, the card doesn't have the best support, but I'm only experimenting at this point.
The WinTV PVR-250 has MPEG-1 and MPEG-2 encoding support, but not decoding.
Thankfully the VIA chipset I'm using - the Unichrome, has decoding capabilities: "Integrated UniChromeTM Pro AGP with MPEG-2 Acceleration". Despite the price (I think I paid around $200) its a nice little mainboard. It has GbE, ieee1394, 2 SATA ports, S-video out, RCA out, and doesn't use too much power. Silent too!