ok, stay away from creative if you are at all concerned with preserving he integrity of the master recording. sb cards resample. who knows if they're bit perfect either (or is that the same thing?)
anyways...
to answer your original questions:
1. rca digitial in (coax) is in the same digital domain as optical digital, just a different format. if you have a d8 you'll be wanting rca (s/pdif) on your soundcard
2. depends...if you get an internal pci card it may be difficult if you've never opened up a computer before. but if you're at all tech-savvy you should not have problems. basically you install the software, then install the card (or vice-versa according to the directions you get with it) and voila, you're set to go. even easier with a usb device such as the egosys u24 or emagic 2|6. there might be some tweaking necessary on your system to optimize performance and prevent dropouts while transferring recordings.
3. yes, i have
i've installed a few into my own, first a sb live card before i knew any better, and then an echo mia card.