If you plan to record movies to watch later and still want to get decent performance out of your Mac, get the
Elgato EyeTV 250+. It has hardware encoding for the MP2 file format. The EyeTV Hybrid depends on the host computers processor to do the encoding.
If you want to record and watch on AppleTV or iPod get the 250+ AND the
Turbo.264 from Elgato. The Turbo.264 handles encoding to Mp4 format so your processor does not have to do the work.
I have both of them and was using them on a Mac Mini. The CPU barely had any additional load while recording and encoding.
If you get your TV via cable service you will not need an additional tuner after the 2009 switch. That applies only to people using antenna.