I used my old Medusas a few times in stereo mode (CD / DVD switch on the older Medusas) with Dolby Headphone on a Xonar D2. It was markedly more realistic and spacious than the Medusas in 5.1 mode. Doing that test completely convinced me that the Medusas' design is hopelessly outdated, compared to virtualised 5.1 over stereo.
If you're getting a Xonar D1, I would recommend throwing £10-15 at a set of Goldring DR50s and another £5-10 quid on a desktop or clip on microphone. I use the logitech desktop mic (non-usb version). They completely blow the Medusas away (although the bass isn't rumbly if you like that sort of thing...).
If you really want to, you can use the Medusas in 5.1 mode with a Xonar but it would be a real waste of a good soundcard.