You don't need to bother with the Mixamp on the PC, unless you somehow find yourself preferring Dolby Headphone to CMSS-3D Headphone. Just connect the Objective2 right to the primary analog output jack on the X-Fi card. (If it's a Titanium HD, no worries; it includes a 3.5mm to RCA adapter cable to save you the hassle of buying yet another accessory elsewhere. However, Creative stupidly didn't color-code the RCA plugs, so you'll need to make sure they're plugged into the right channels and not reversed with a bit of testing.)
The X-Fi Titanium (non-HD) models have full-size Toslink S/PDIF ports with no need for adapters, while the X-Fi Titanium HD has mini-Toslink input and output on the RCA line-in jacks (so you do need adapters for those jacks, or you could just use the included Toslink to mini-Toslink cables).