A 1.4 Athlon is more then capable of running mythtv.
The hauppauge card is an excellent choice, I am currently using a PVR 250 in one machine and a PVR 150 in another.
Both run perfectly.
The file size seems about right.
A 1 hour of television would be a few gigabytes.
I have never personally seen the error you are recieving, so the only help I can offer is what google dug up.