Ok, I hope this is the right place for this. New to MythTV and trying to build a box from extra parts. I have:
Athlon socket a 1200mhz
512-1gb ram (whatever is needed)
seagate 160gb hd
Asus or Msi(raid) mobo
I only want to record one show at a time, no need for viewing while recording.
I am not sure what card to get or if it will work. I was looking at the Hauppague WinTV 401, but I want to record comcast digital cable channels (not HDTV). Will this hardware work or will I even be able to record the signal?

I would be grateful for any help or pointing me to where I can find the answers.

First off 512mb of ram should be plenty, you have plenty of HD space and the 1200 mhz should work just fine. Personally I like the Hauppauge pvr-250, it has a hardware mpeg encoder which makes life easy for you machine. Also it records to disk in mpeg format. Make sure get a decent nvidia card with composite or (preferred) svideo out if you want to hook this up to a tv. Does your cable box happen to be a Motorola with a firewire port on the back? If you have the right model you may not even need a capture card, you can use firewire to record from the cable box. The other option is to get an IR Blaster and have the mythtv machine change the channels on the cable box. The output of the cable box will run into the pvr-250 and you will be all set. I think I covered it all, hollar if you have more questions.

The PVR-150 model 1045 (non-MCE) is both cheaper and includes a supported IR blaster which you may need to drive your digital cable box. You can probably get this to work with a YUV software encoder card, but the PVR will simplify your life enormously and ensure that you don't have CPU problems.

If you haven't found the wiki yet, start here: http://www.knoppmythwiki.org/index.php?page=PickingComponents It's been updated recently. Also see the HardwareConfigs page there.