My first post here, hi to you all:)
I am surpised nobody has mentioned this new heapdhone/stereo card - Xonar Essence STX - from ASUS.
Built in High impedance headphone amplifier and high grade components boasting a SNR of 124dB. That is a very nice SNR and I do think it is definitely capable of making your PC on par with some of the much more expansive CD players or headphone amplifiers.
Here are some of the reviews that I've been collected from google search, and they're all very impressive...