An internal TV card uses a PCI slot - AGP is for a graphics card.
If you have spare PCI slots available then the internal type will probably produce better results display wise.
I use a Twinhan Freeview DTB-ter Digial + Analogue (D+A) PCI TV card which can receive Freeview and terrestial TV and radio stations, plus FM radio, and is also HD compatible.
Twinhan also produces a straightforward Freeview version (i.e. no facilities to import games console and VCR signals for example) in the DVB-ter. It can be used with a modest CPU.
Other companies produce similar cards, of course, but they tend to be a little more expensive.