Depends what'cha gonna use the computer for and how much of a sound junkie you are TBH. If you just want to be able to hear your games, music and watch the odd DVD then theres nothing wrong with onboard sound paired upto a nice set of speakers.
If your gonna invest money in a soundcard, then I wouldnt do it on the cheap as with the quality of onboard sound these days, you may actually go backwards by chucking a £20 cheapo card in the machine. Its not even like you need one for Optical or Coxial in/out anymore, as allot of the higher end mobos come with those as standard. Me thinks, if you need to ask, your probally gonna do fine with onboard!