I'd be weary of the USB TV tuner from ATI. I know that it sounds good and is one less PCI card to worry about if you are overclocking but from the reviews that I have read, the usb tuner is not nearly as good as the PCI cards. I decided to get what was on sale and bought the ATI TV Wonder LE (light edition). It was cheaper than the others and I am not disappointed with it's performance at all.
Tom's Hardware has a good article on TV tuners, check that out. Also check out hauppage (it's something to that effect) - they have cards from $50 to $400 and all of them are sweet (the 400 is HDTV with recording and all these other features) - naturally, keep in mind that watching TV on a computer is a lot like DVDs - I seriously recommend a 21" or at least a 19".