sound cards don't amplify the signal, a signal amplifier amplifies the signal.
alpine head units average output 4v
eclipse head units average output 8v
sound blaster average output 2v per this website (audigy 4):
http://www.soundblaster.com/products...ct=10853&nav=1
the higher the output in voltage, the less an amp has to work to amplify the signal, thus making it more efficient. as you see, the sound blaster is at the bottom of the list. it won't fry the amp, you just need to turn the gain up more than you would over a regular head unit.
it's not the case that the sound blaster "amplifies" the signal, it's just that it's not as efficient. my recommendation- tell the guy at circuit city you want the amp (get the warranty) and flip him the bird for being a dip**** because you had to make 2 trips to get the amp.