Onboard for everything, though I do have a couple cards with Dolby Digital Live encoding
that I may eventually use for converting PCM/WAVE output to an optical out going into an external
DTS/DDL type receiver/amplifier.
I suspect if I searched I could find a software encoder scheme for LINUX/UNIX that would do realtime
DTS/DDL encoding with even the digital output on a vanilla onboard sound chip, though, so there's even
less reason to use a sound card.
Basically the only reason to use a sound card IMHO is if you want a high end one with really nice
quality ADC/DAC support better than onboard, and more total input / output channels than onboard.
I hate Creative Labs now due to their horrible driver (and often product) quality and lack of
good open source os support (LINUX, BSD, Solaris, ....), and lack of published specifications to enable
driver support / development. I'll never buy any of their cards again.