[rdb1]

I am looking to get some on ear or over ear headphones. Open or Closed. Budget is up to £200 so I would like a mixture from say prices from £100 - £200. Mixture of music.
 
Possibly looking at Sennheiser 598 or 650 at the most. - Would this soundcard work okay at driving these or should I look at A Cambridge Audio Usb DAC as I have one of those too.

 

[mindbomb]

fostex t50rp. the soundcard has high output impedance with no headphone amp. Planar magnetic headphones don't care about that.

 

[PurpleAngel]

  I am looking to get some on ear or over ear headphones. Open or Closed. Budget is up to £200 so I would like a mixture from say prices from £100 - £200. Mixture of music.
 
Possibly looking at Sennheiser 598 or 650 at the most. - Would this sound card work okay at driving these or should I look at A Cambridge Audio USB DAC as I have one of those too.

 
Hopefully you disable the motherboard's on-board audio, in the BIOS, when you installed the Xonar D2X.
 
The headphone (line-output) jack on the Xonar D2X has a high output impedance, which might make headphones plugged into it have a slightly bloated (louder, less detailed) sound quality.
Getting a headphone amplifier, like the FiiO E11K ($60 USA), could be used with the D2X and should improve the audio quality of any headphones you use with the D2X/E11K.
 
Headphones like the (120-Ohm) AKG K612 Pro (102 GBP) might do fairly well plugged straight into the D2X.
You could always get the K612 Pros first, then have the option of adding a headphone amplifier.