The HD650 are a good headphone, but they require a headphone amp and would probably not sound too satisfying straight out of an X-Fi.
I'd have to agree with that. Driven properly the HD650 sounds absolutely stunning, but it really does need a dedicated headphone amplifier to get the best out of it. Plugged directly into a sound card it will probably sound muddy.
(I use HD650 'phones myself. My headphone amp is a Graham Slee Projects "Solo". If you were in Europe, especially the UK, I'd recommend it heartily, but the Solo is British-made and the strength of the pound against the dollar at the moment would make it a questionable purchase in the US.)
Beyerdynamic DT-880 and AKG K701 are also very fine 'phones, but (again) you'll need an amp to get the best out of them.
Grado RS1 or (if you can't afford them) RS2 would possiblybe a good bet - they're pricey but they aren't so amp-dependent.
If you are considering buying a headphone amp as well then there's one other option you should consider, which is an electrostatic system from Stax. These look very expensive at first glance, because you have to buy a dedicated driver unit, but the price of 'phones plus driver actually compares quite well with high-end dynamic 'phones plus an amp.