If you have a cable or fios box or dvr, you could try to cap the unflagged channels through firewire.
HD tuner cards will only receive OTA and QAM which is basically limited to you locals. I'll let you know right now with the digital transition, a lot of broadcast stations have changed from UHF to VHF which has made indoor antennas almost useless. I wouldn't recommend getting a tuner unless you have cable or fios QAM. Either that or a rooftop antenna that you have verified works with a TV tuner. The only way I got signal from VHF channels with my indoor antenna was sticking it out arms length outside a 2nd story window. So recording OTA for the newer VHF channels is pretty much done for me.
Hauppauge HD PVR can record from any source with component inputs, but I doubt you would be able to playback the h.264 smoothly with a P4. Maybe you could get some stills from it, but playing the video right would be pushing it.
Read this at AVS forum and see if you can narrow it down to what you want to try out. If you have any specific questions after that you can post here, and I'm sure someone will try to help you out.
http://www.avsforum.com/avs-vb/showthread.php?t=1061644