Personally, speaking from experience, I have dumped my Marantz CD5400 for my computer. The biggest improvement in my computer chain I obtained was when I replaced my Chaintech AV710 to a EMU 0404. Just for reference, a Marantz CD5400 retails at 230usd, and an EMU0404 retails at 100usd. Of course there is the hard drive and the casing and the PSU to take into account,but at least here the above is a simple reference point for you.
Next advantage would be the convenience factor. If you do what most head-fiers do-- rip their collection to the hard drive (instead of what I do-- run the CD in the CD-RW drive), then you would 1.be able to access your music collection rapidly 2.avoid or lower the amount of errors
Third advantage is, I guess, a soundcard would be easier to mod compared to a CD player.
Feel free to correct me if I got some of the informaton wrong.