Dual Link DVI

If I understand your question correctly I can tell you what little bit I know.

1) Dual Link DVI allows you to set up use to separate monitors with both monitors connected to 1 pc. I have a 25" and 21" monitor connected to my new build pc. This allows me to work on 2 separate pages or projects side by side. Like I can search the web on 1 monitor while watching Live Tv on the other monitor.
Bascally to me its almost like having 2 pc's.
Older video cards have 1 VGA port out and DVI port out.

"most" monitors have 1 VGA port in and 1 DVI port in. when I was shopping for a new monitor I wanted a monitor with 2 DVI inputs. The few monitors out there with 2 DVI did not impress me.
On my 25" monitor I have 1 pc connected to the DVI port and another PC connected to the VGA port. With a button on the front of the monitor I can switch back and forth between 2 separate pc's.

2) No I do not think "all" cards have Dual Link DVI. Most of the new card do have 2/DVI.

3) I have not tested the theory that DVI is comptable to HDMI. But I have seen on newegg.com
dvi to hdmi converters and I have a dvi to vga converter that is in use.

Use wikipedia for such common asked question, for one, dual link dvi is NOT ABOUT HAVING TWO DVI PORTS.

Dual link dvi is one dvi socket which has two signal generators in it so that it can, with a few more pin connectors allow twice the bandwidth of a single link dvi socket.

That can allow either higher res's or more Hz depending on what is desired. basically twice the info.

Practically all gfx cards have had at least one dual link dvi port on it.

HDMI Is based on the dvi standard so yes they are comparable.