Hi,
Might be worth checking that your motherboard will support 24-bit playback / operate WASAPI etc.. If it does and you don't have any requirement for hardware sound processing for things like gaming (EAX etc) then I should think you'll be fine just using the COAX, USB or SPDIF that is on your computer's motherboard.
I opted for the Creative Labs SB X-FI card and run it to a DAC via TOSlink.
I heard/read somewhere that not necessarily all soundcards/motherboards support WASAPI / 24-bit playback which is something I was specifically after and the SB X-FI series are one of the cards that does. Though this may well be nonsense. I also assumed electrical intereferance from the PC would be less using optical medium, again this might be nonsense or make an inconsiderable difference but the current setup certainly works fine!
If you do go for a soundcard there are Creative SB X-FI PCI cards on sites such as play.com for £35 - £55 including delivery and support "Playback: Stereo: up to 24-bit/96kHz, Surround: up to 24 bit/96kHz, SPDIF: up to 24-bit/96k". There are obviously other cards which provide this however I've always heard good things about the Creative Labs cards and are the only ones I can recommend you from personal experiance.
Hope this helps..
P.s. Once your all setup with your hardware my personal recommendation is to rip CD's to FLAC format using a program called EAC (Exact Audio Copy). Then playback via any player that supports WASAPI mode (Foobar,Winamp etc (you'll know when this is active becaues it will cut all other sound generated by the computer and probably throw up playback errors if you happen to have something like spotify running)). Good luck!