You won't have any noticeable change in quality if you have a good sound card, with good analog-to-digital converters. It will degrade quite a deal using a $15 POS card, a little less with your average SoundBlaster, even a little less with a good Turtle Beach Montego card, and hopefully not at all with a (relatively inexpensive) professional card in the $150-$200 range.
If you want to keep the tracks separated on the CD, you have to burn it as a data CD. This will allow you to have all the tracks backed up, to do a mixdown later on. You wouldn't be able to play them on a CD player, though.
Or, you could mix down on your 8-track and just run a stereo out (line out? monitor out? don't know what you got) from the machine to the sound card's inputs, and use some kind of digital audio recording software (I recommend
www.goldwave.com) to record wave files (plain audio files on your computer). The wave files can later on be burned onto a CD in audio CD format (using a CD burner and proper software for the burn process, of course), and (hopefully) be played in any CD player.
But you can't replace the tape in a tape recorder with CD's, if that's what you meant.
