Well, according to:Fujitsu-Siemens AMILO M3438G - Gaming Notebook - TrustedReviews
You have an spdif/optical line out. You probably have to disable the headphone out, and make it operate as an optical out.
A good DAC which I'm about to audition is a Keces DAC. It has an optical jack for you to use, otherwise, you can get a USB DAC, like by purchasing and Alien DAC which can be found here. or you can also use the Keces unit as a USB dac. If you want high end, you can go Lavry or Benchmark.
For the ease of convenience I would go USB DAC simply because you can use it on any computer and use it in the future if/when you decide to upgrade PCs.