The Elgato EyeTV HD is by far and away your best option. You get Full HD on your iMac in it's own application (included EyeTV software) allowing you to work or surf the web with a TV window open in the corner of your screen. OR you can just make the video window full screen and sit back and enjoy.
With another HD cable box, you just hook it up to the EyeTV HD via component cables and hook up to your iMac via USB. It's really awesome and well worth the $199 price tag. The software is amazing as well and let's you record and edit TV and makes it super easy to export those recordings to iTunes for use on an iPhone or iPod as well.
By setting your HD cable box to 1080i, you also get 1920x1080 resolutions from this thing while the DisplayPort In jack is limited to 720p. Those extra pixels are definitely noticeable on a 2560x1440 iMac display.