IMHO, you only need DAC if you have not got one already or you are SURE yours is poor. Otherwise get and amp only and plug it into line out on your PC. AMP-wise I would recommend Topping A30 but there are cheaper and more expensive around. All goes down to how much you wanna spend, what do you expect from it (performance, looks, versality/mobility etc...).
Check also those:
- Maplin HD 24-Bit 192kHz 7.1 USB Sound Card With Front RCA Stereo Outputs Audio
- Creative Sound Blaster Omni Surround 5.1 USB Sound Card
- Creative Sound Blaster Z (or ZxR if you have got a lot of spare cash)
- FiiO - E10K (low power; recommend impendance of headphones up to 150 Ohms)