Re: X-fi to 5.1 Receiver?
Use the analogue outputs with 3.5mm stereo jacks to stereo RCA plugs. You'll need three for 5.1 sound. One will be for front left/right, one for rear left/right and another for centre and LFE (Sub).
Or you could swap your card for one of those Prelude 5.1 things which will do 5.1 for games out of the digital out.