Onboard sound can be noisy, especially if the board layout/circuitry is not well done. PCs are notoriously noisy places (which is why many pro interfaces have long isolated their noise-sensitive bits in outboard boxes connected via an umbilical).
Dunno about Sound Blasters (haven't used one since the ISA days), but do use a HiTec card in my gaming/disc watching/general purpose box. Think they are branded Auzentech (or something like that). A bit expensive (maybe $70 back when iirc) but it is very nice and clean sounding, has analog and digital I/O and the op amps are socketed so as to be easily upgradeable if desired for even better audio (never felt the need).
One thing you might try, if your onboard sound has an S/PDIF out, is using that connected to speakers (or an A/V receiver or a DAW interface) with an S/PDIF input (n my setup, my cheapo surround speaker system has both analog and digital (coax and optical) inputs).