Erm... Lots.
I was pretty happy with EAX representation on Assassin's Creed and Bioshock on Xonar and 5.1 is a given. Crysis and HL2 Series / CSS / L4D / other recent source engined games are all software based all features can be enabled on Xonar, and sound pretty good.
It's rare these days to find a game that doesn't do 5.1 very well. DTS connect is irrelevant really. It's just a way for you to connect your speakers via a digital connection.
If you find that 5.1 isn't working properly, just restart the game and hit the GX button on the Xonar control panel.