I just spent a lot of time web shopping to decide which set was the best set, price aside. If you can afford them the Psyko Audio guys won the best new innovation award for CES 10. Check them out http://www.psykoaudio.com/ . I opted for the G35 to match all my other Logitech G gear, still awaiting delivery tomorrow to see how they perform.
G35 will totally ignore the soundcard, so your ASUS Xonar DX is now going to waste. USB headphones contain their own unique audio driver that will pre-empt the soundcard in all cases.
Would have gone with the Tritton anyway; 5.1 via optical, dolby decoder module, etc. Alternativly, could have gone with the PC350's and simply used the DX's Dolby Virtual Headphone to get virtual 5.1 on the headset.
Take a quick look through Head-Fi's Headphone Buyer's Guide for a list of great cans.
Looking at the price range of your other choices and your unamped sound card, I would probably recommend Grado sr80's or Sennheiser HD-555's.
Both cost around $100, should work fine with your sound card and both will sound much better than any of the headphones yet listed.
If you need a microphone, using a simple Clip On Microphone will give you the best of both worlds.
What ever you end up getting, DO NOT get a pair of USB headphones!
They have their own onboard CODEC (basically being their own sound card) and will not use your Xonar's excellent DAC.
I would also recommend against a pair of 5.1 gaming headphones.
Everything pair I have seen reviewed has gotten pore marks for both sound quality and surround sound accuracy.
Instead, stick with a good pair of stereo cans and your Xonar's excellent Dolby Headphone implementation.
I have been gaming with my AKG's for quite some time now and I can say that it makes the games feel almost as good as using a dedicated 5.1 surround speaker setup.
Yes, you can usually hear quite a bit more with headphones than a cheap surround setup.
Still, using a dedicated sound card and higher quality cans would be a massive step up from a USB headphone setup....