Hmmm, I've personally had better experiences with my SB Live! 5.1 than with the onboard sound on my AI7. Much less (actually none, as opposed to a noticeable amount) audio lagging and/or crackling when in the middle of a game, etc.
As for sound quality, I haven't noticed a big difference between the two (SB Live! may have been a bit better). It may make a difference that I'm using 6-year-old Altec Lansing 2.1 speakers, but that's always been plenty for my needs. Of course, your mileage may and probably will vary, but from the perspective of games running smoothly, I'd say go with a sound card.
(OTOH, I've heard that installing a SB heatsink can sometimes help with onboard sound issues. Not sure if that applies to lagging sound or not.) -- Paul