Just stick in a tuner card to any of the PCI slots. If you do not want to open case out, u get a external USB2 tuner.
I prefer PCI based because it is more responsive. If you want to use TIVO like features, get one with a hardware encoder so that CPU utilization is lower.
ATI Theater 550 Pro based cards offers excellent video quality and FM Tuner. However the PowerCinema Software that comes with it is crap. (can't even change the folder it records to) So be prepared to shell
out for Beyond TV.
http://www.anandtech.com/multimedia/showdoc.aspx?i=2234
Much cheaper to get Sapphire Theatrix or Powercolor than ATI TV Wonder Elite. Only difference is the remote. ATI remote is more universal that works more like a mouse + remote features while
If you use Multimedia a lot, skip Beyond TV and use Microsoft Media Centre Editiion 2005 (Windows XP + Media stuff) Usable interface that works well with a remote.
MCE 2005 also works with more than 1 TV Tuner. That is you can record and watch another channel at the same time. Access TV guide automatically, etc.
http://www.anandtech.com/video/showdoc.aspx?i=2393
http://www.anandtech.com/multimedia/showdoc.aspx?i=2240