Is this for gaming too or just HTPC?
For just HTPC you could get a mobile XP chip instead of an A64.
For HTPC having a card that does hardware MPEG2 encoding (like the Hauppage PVR series) will make recording more reliable than software, and cut way down on the CPU power needed.
Hauppage says only a 1.x GHz P4 or XP equivalent is needed for simultaneous playback of one part of a program while recording another part. For just recording, the card does almost all of the work.