The difference is in the panel. Most brands, including Dell, are trying to satisfy most consumers who big, fast, and cheap. Good color but great color.
FWIW, I read good things about the Lacie monitors regarding color matching, along with Eizo and Apple. But I cannot offer an opinion, as I am colorblind. Lots of good general use panels out there, but most are not made for precise color matching.
As for HD thing, to the best of my knowledge, that is only related to video. Don't sweat it.
Video is tuff, as Rick said. Only a couple official choices. Beyond those, you can consider a flashed PC card, but that is a whole other challenge.
If you want a good Apple monitor.....sell that G5 and get yourself a new iMac.

. Faster, quieter, good panel, good price. full HD video ready. As long as you are ready for 10.6........I think you will need CS 3 as a minimum.