Remember that the iMac’s display will approximate the color gamma and contrast of most off-the-shelf ATSC HD displays (in system prefs/displays you can set the color gamma and white balance), but that also when cutting you’re only seeing a thumbnail-sized low resolution version of the HD Video, which is acceptable through the entire process up to the color balancing/correcting part. To truly see this, you would need to be able to view the content through an externally connected video upconverter (connect to FireWire port), and Black Magic and AJA (and Matrox) are all companies that sell that external box. You would then need to connect it to a quality Reference Display (pro specification) as with Panasonic or Sony’s pro Broadcast lines, to accurately measure the high-res output.
Even then, I would check to see if the iMac can even support that kind of output. You may just be delaying the inevitable-upgrading to a Mac Pro (tower), in which case, you could look to the PCIe HD video card options of what I mentioned above.
With all of this, I’m assuming you’re referring to Professional quality production; if you’re just shooting with a consumer-level HDV or AVCHD camera, none of that will really matter. In the end, if you’re not authoring onto Blu-Ray (no one on a home platform can yet-the software and hardware are not quite there yet), your content is still being dithered down to 480p DVD NTSC….it’s analagous to recording in a million dollar studio only to make your final master burn onto a CD-Audio versus a DVD-Audio (if you’ve ever heard a DVD-A you would appreciate the comparison).