I haven't purchased my TV tuner card yet - but I've done alot of research - and think I have a grasp pretty well.
You have to first choose between Digital and Analog.
If you are using CATV - you have a choice - if you're using Antenna - everything's Digital. So your choice of input in your card would depend on your source.
If you are taking a source direct from a cable line ( Pre-Cable Box ) - you'll order either Analog / or Digital signal from the Cable company to feed to your Television.
CLEAR QAM - is generally needed in order to display your CATV source direct from cable up to about 100 - 125 direct from your CATV company. Source would depend on if you ordered Digital or Analog from you CATV company. Many cards wil take one source - and split it - and give you two tuners to watch 2 separate channels from this source. You can take a source direct from your cable box ( what I'm probably going to do ) for an analog signal if you want to use your cable remote.
ATSC = is analog broadcast signal. I don't think this means much since the digital changeover
NTSC = is digital broadcast. It's the only way you can get a High Def signal to your Television right now - as much as I can understand. But you need an over air antenna to get the signal.
AverMedia looks like the most bang for your buck - from what I have read on NewEgg's reviews - Works well with Vista Media Center and Win 7 - not sure about Win XP so much.
( This is a good price - usually goes for 85 bucks )
White Box = Just a card. ( you may or may not want a Windows Media Center remote with it - they sell for around 20 bucks )
Retail box - usually come iwth Remote / Tuner card / and cheap antenna.
Powercolor ATI-550 looks like the cheapest good picture technology for Analog signal. 25 Bucks on Amazon for White Box version. The technology has been around for a couple years - but from what I was able to gather - the best tech for clear pic Analog signal.
http://www.newegg.com/Product/Product.aspx?Item=N82E16815309007
Here's a website that's useful.
http://www.hdtvtunerinfo.com/
When I asked here - and at Tomshardware - I didn't get many replies on TV tuners.