While you're at it, check out these videos recorded with CMSS-3D Headphone and Dolby Headphone. They'll give you a good idea of what to expect.
Since the Presonus Central Station is also a DAC, I'd suggest picking up an X-Fi Titanium used or refurbished on the cheap. (Unfortunately, Newegg went up to $75 from $45, so I can't recommend them right now.) Then connect it via S/PDIF, and you'll be using the Presonus Central Station as DAC and amp while the sound card serves as a cheap DSP for binaural surround processing (and EAX, if any of your games use it).
Oh, and avoid the XtremeAudio cards at all costs!