Well my onboard ASUS ADI2000 can power them very nicely, my Onkyo Receiver does much better tho....
The Denons are only 50ohm, but they can go all the way up to 130dB so be careful (rule of thumb if it tickles your ear its too loud.) My HTC can even power them (just.) The advantages of amplification, over your soundcard, will probably be minimal, like <1% clarity, and volume limits. So I wouldnt worry about it. Just if you do buy them, its worth noting they are slightly picky with amplification (old Onkyos are by the far the best bang for buck in my opinion,) so take the Denons with and test on the spot if possible, or read up.
Damn i just read up on that card, its more then enough to power them, so the volume shouldnt be a problem at all.
Also you can swap out OpAmps (and they are cheap) to fine tune your sound preferences if you want, it will probably make up that <1% difference with the right combo.
EDIT: Its worth noting that even though the Denons are closed back, they leak sound like a rusty hooker... But its worth it in my opinion, they are comfortable, sound amazing with all genres, especially when listening to flacs, they are durable, and by far one of the best things i have bought for myself.
Edited by WiR3D-kNiGhT - 10/22/11 at 3:03pm