Looks like you already have the 840C. If you are happy with it for CDs, you should be equally happy using it with a PC. I tried it in my system, and the sound from a CD and the PC was pretty much the same. I found it a little too detailed and "digital" for my liking but I know a lot of people like it. I ended up with a Musical Fidelity A3-24 DAC, which might be a good fit with your system.
For a USB to S/PDIF converter, I am currently using a M2Tech HiFace ($150) which is remarkable for the price, although it is a new unit and the drivers may not work with all players yet. I am used it with Wind 7 with J River Media Center, which is a great player. The 840C supports up to 24 bit 192 KHz input, so your USB converted should also if you expect to use high res tracts. A lot of the USB converers do not go above 48KHz, so that is something to watch for. The HiFace goes to 24/192.
Once you settle on a player and an interface you should be able to get sound comparable or better than what you get from CDs.
For a USB to S/PDIF converter, I am currently using a M2Tech HiFace ($150) which is remarkable for the price, although it is a new unit and the drivers may not work with all players yet. I am used it with Wind 7 with J River Media Center, which is a great player. The 840C supports up to 24 bit 192 KHz input, so your USB converted should also if you expect to use high res tracts. A lot of the USB converers do not go above 48KHz, so that is something to watch for. The HiFace goes to 24/192.
Once you settle on a player and an interface you should be able to get sound comparable or better than what you get from CDs.