from my testing - the ONBOARD sound puts out only .5v
you will want to boost that voltage to an acceptable level. I used an Arc Audio PEQ-7 which is an eq, it boosted my voltage to 9v. Doing so will give you a much cleaner sound. You also won't have to worry with trying to find an amp that matches your output voltage of your sound card.
In general, i think that you are looking at this with too technical of a mind...lol
just pick an amp that you like, and get something like an eq or other that will boost your voltage level to an acceptable level...