an external or internal tv tuner card.there are options for hdtv tuner also from ati.if you want to record tv shows as well to your hdd you might want to consider a pvr card.a hauppauge pvr 150 should do you well for about 70$ but thats only if you want to record tv shows.ati hdtv wonder for hdtv for about 160$ and up.
hauppauge deifinetly, but if want dual tuners so that u can record 1 show and watch another or just record 2 at the same time then ull have to go for the hauppauge mce500