I've been pretty pleased with the ALC888 HD audio that came with my ASRock board. Bought a Chaintech AV-710, set it up to use the high-quality stereo output through the Wolfson DAC, and couldn't really tell a difference. Stuff that I thought a better soundcard might fix are either problems w/the MP3 encoding (am just not willing to do FLAC for space reasons) or speakers (Klipsch Promedia Ultra 2.0, maybe not the fanciest speakers but they're not crap either). Got rid of the Chaintech to free up a PCI slot.
You may see a big improvement with an older codec like AC'97, but based on my experiences and what I've read, recent onboard HD codecs aren't supposed to be too bad.
Then again, I'm not a huge audiophile, so take my $0.02 with a grain of salt.