The DGX supports 5.1 via analog. It can pass through 5.1 formats (Dolby/DTS) over optical, but can not convert un-encoded audio streams to either format. The DSX supports DTS encoding, the DX supports Dolby encoding. If you plan to use optical, you might want to cancel your order and get one of those cards.
Alternatively, you can use analog, though you will probably need to do a bunch of TRS to RCA conversions, making cabling a bit of a hassle.