Thanks very much Jason. Okay I'm still in two minds.
Option 1 - Get a DacMagic or rDAC and also headphone amp (not sure which). The gives me a dedicated Headfi rig. Its also more flexible because if I decide to use the DAC ocasionally on my HiFi, this will improve the SQ here too.
Option 2 - Get Asus xonar (the ones without fan) or Cantatis Overture 192. Limits me with drivers/software etc. Good because of price but cannot use the DAC to upgrade the HiFi sound as its in another room.
I was thinking if I get the rDac it has a good USB connection, so could use this and by-pass the onboard sound altogether. This may give better results than using the coaxial output.
I'm pretty set on buying a pair of AKG 271Mk2 headphones, so maybe I should just see how I get on with these to begin with and then look at DACs later.