if you are someone who really cares about sound quality, then you should definately get a sound card. personally, i cannot live with onboard sound. My music/games/movies all sound lifeless with onboard (but that is just my opinion because im someone who gives a crap about sound). anyway, onboard will use very little resources, so little you will not be able to notice.
if you want my suggestion, i'd go for an audigy 2z or if you have the money, the xi-fi extreme music (these cards are from creative).