i use the 945 gclf(sp?-- the basic one from mp3car's store).
wheather or not onboard sound is good enough is personal preferance, if not, add a soundcard.
plugins for crossovers, etc. is more of processor intesive process, and is not so much related to the audio card in use. but you can setup vst hosts to manipulate the onbord audio outputs if needed.
like i said, i am using the 945, with a integrated atom dual core, and really don't have any issues(a bsod occaisionally, but i am sure that it has siomething to do with my mo.bo. and sound card not getting along). i am also using audio mulch for audio processing (this week i am using 3-karmafx 31 band eq's, and 3-karmafx crossovers for my 3-way setup).