I have just installed my Asus Xonar Essence STX II sound card. Using the analogue out direct to my pre-amp I am struggling to tell a big difference between this and my external DAC set up.
Further listening is required, but the DAC on the sound card seems very good. There is a big price difference between the two set-ups for just the "HiFi" part.

The PC I am using is one I had lying around and is an old Intel Core Duo E8400, on an Asus P5K motherboard fitted with 4GB of RAM. It still has a couple of fans that I am thinking of getting rid of if I can.
The external DAC is a CAIMAN SEG with MCRU linear Power Supply and the Halide Design SPDIF bridge.

There is a small difference between the two, but I can change the op amps on the sound card to tweak the sound for the better.
It is really making me think about getting rid of the external DAC set up.