Chances are you are using an onboard soundcard if you hear floor noise. Otherwise, a very cheaply-made soundcard. Either way, stick to something from Creative (if you're a gamer), Voyetra (purely music SQ preference), or M-Audio (for reference-quality sound). NewEgg should have everything you'd need. If you're interested in purchasing a card and need some help/budgeting, give us a hollar.
HOWEVER, there's the chance that the noise you are hearing is from a cheaply-made amp/power supply in the speakers you are using. Best way to see if it's the soundcard is to hook up a nice pair of headphones to the speaker jack on the soundcard. If you hear noise, then it's probably the soundcard. To see if it's the speakers, try another sound source, such as an iPod, Palm Pilot, laptop, CD player, etc.
Good luck!