The best soundcard for under $100 is the ASUS DX. If you want something more "gamer oriented" [and yes, thats meant to be sarcastic], pick up the Auzentech Forte for around $130 or so. Just don't buy Creative...
And anyone who claims that a good soundcard [Good implying non-Creative] will sound the same as onboard needs to have their ears waxed. 😀