You're in luck, it appears you do have 5.1 analog in. Refer to page 8 of the operating instructions and replace the DVD player at the bottomw of the page with your PC. The audio cabling on the left side of the diagram is the important part.
All you need is the cheapest X-Fi and then purchase one of these cables, then pray it is long enough.
All you need is the cheapest X-Fi and then purchase one of these cables, then pray it is long enough.