It depends on what monitor you've got and what types of signals and input formats it can take. Plus what about speakers? (built into it?)
Tbh, I'd start with the 'signal' you have (basic analogue/freeview quality aerial/satellite feed and go for a card/box/USB stick that will take the signal and use your PC to convert it to display and hear through your monitor and speakers. It'll also give you a PVR built in.
I bought a cheap but reliable USB freeview stick (DVB-T) off ebay for £10 delivered, fine tuned my aerial for the best freeview signal and use DVB ViewerPro to watch TV and record/timeshift TV. I just love the desktop TV feature.