I guess the move to more cross platform development lends itself to software audio.
I don't think I have a single game installed that uses OpenAL actually, all I am playing at the moment is GTA4, Guild Wars (to help the GF) and Team Fortress 2. I don't think I would lose anything by using onboard sound, maybe slight FPS decrease, depending on what the soundcard is actually doing.
The main reason I was asking is it sounds like in my current situation, I could remove my X-fi, go for a HD5770 card running HDMI to a HT amp, and then just use the headphone out on that, should be equally as "low-fi" as I have now, just with different kit. WASAPI works for HDMI out too yes?