You could get the Creative 5.1 Live Soundcard, which has Digital(analog) output (Dolby Digital 5.1 only - i believe), and connect that to your amp.
Or get something like a hardware DVD decoder card based on the Hollywood plus Chipset (e.g.
www.scan.co.uk sell them for around £50), and that outputs DTS & DD5.1 out on Analog, is multiregion, has TV OUT and does Hardware DVD playback, so u don't even waste CPU cycles playing the DVD...
terra cineview dvd decoder card it's regularly on their Today Only page too..