In simple terms, the Titanium HD has stereo output with similar quality components when compared to the STX, along with all the goodness that Creative has when it comes to games. If gaming is your main purpose, the Titanium HD is the way to go. SQ is also up there for when you want to listen to music or watch movies, you won't be underwhelmed by it compared to lower end previous generation Creatives.
Running your D2000 through a PA2V2 connected to a Titanium HD would sound sweet, though I'm not sure D2000 would be the best for, let's say, competitive gaming like FPS where positional cues need to be accurate and clear, seems like bass gets in the way of that.
Bottomline, Titanium HD has at least the same SQ as Xonar STX, much better gaming performance, but stereo output. For a gamer, the choice is clear, but it's up to you 