Depend on the card but,
Several cards have Analog inputs and outputs together with the DV ones. This enalble you to capture from more sources and output directly to VHS.
One more thing that is probably the most important it give you a option to monitor your work on a normal TV during all stages for work.
Second thing is real time options.
Let's say that you have two video clips on the time line and you want to put a transition betwin them.
Normaly you set the transition and then need to render the video in order to see the resoult. If it's not good you make changes, render, chenges , render untill you get it correct. With a Real time system you can just put the tansition/Filter/Color effect and play out. This save time as well as drive space.
Some cards offer excelerated export to various file formats. for example you can get a card that do some of the final DV to Mpg2 encoding using hardware and this make exporting to DVD or VCD much faster.
Depend on the card, and as the card is more expencive it will offer more features in real time or excelerated features.
Install of the card is very much depend on your hardware with emphasis on main board. Some capture/Edit cards just don't like some chipsets or configurations.
Normaly you can make it work very nice with little tweeking if any. My Pinnacle DC2000 was working perfect from start on my A7V.
Intel / AMD - <A HREF="
http://www.llnl.gov/asci/news/white_news.html" target="_new">IBM are still the best</A>