The reason why you haven't found an answer yet is that there is no simple answer. There's a lot more to how sensitive a headphone is than a simple impedance rating at both ends, and few people on Head-Fi ever realize this.
-Some headphones have crazy impedance curves over the frequency response (Sennheiser is notorious for this), while others (usually orthodynamics) don't and tend to sound consistent out of sources that don't have an extremely low output impedance.
-Being driven loudly doesn't necessarily mean being driven well.
-Impedance itself is not an indicator of sensitivity. The AKG K701/K702/Q701 have a relatively low impedance, but no one would dare suggest that they're a sensitive, easy-to-drive headphone because of that. If anything, they're notorious for being very demanding on amplifiers.
My suggestion is to just try a headphone straight out of the sound card to begin with, and if it doesn't sound good enough, then consider adding an amp. (Unless, by some chance, you get an electrostatic setup down the road, where the amp isn't optional.)
