It is at least ten years since onboard sound became good enough to obsolete discrete sound cards yet I have just bought an ASUS Xonar DGX for my PC. I have no illusions that this €30 card will give any noticeable improvement in sound quality for modern games. I bought it because it comes with an emulator for Creative Labs EAX technology. EAX was the dominant environmental sound method used in PC games until the advent of Windows Vista. EAX was proprietary to Creative labs but every sound card manufacturer had some form of EAX compatibility back in the day and a huge number of games used it . Vista and all later versions of Windows changed the way sound is handled and EAX no longer works. Modern games achieve rich audo environments through other means but older titles that relied on EAX are left high and dry.
I enjoy playing older PC games and it always irritated that I could no longer get the full experience due to the lack of EAX. Many of these games relied on EAX for positional audio as well as for envronmental effects so the game's soundscape is much reduced without it. There is a software emulator available called indirectsound that can be used to restore 3D postional sound but it does not implement environmental effects (eg the way things sound differently depending on the environment around you).
Asus GX2.5 capability claims to emulate all the features up to EAX5.0 and because it is pure emulation as opposed to a hardware/software combination like Creative's own "Alchemy" it should work for every EAX game. I haven't had much opportunity to try it out yet but I will report back when I have had a chance to test it. Here for reference is a review comparing Asus GX2.0 (an older version) agains Creative's Alchemy: http://techgage.com/article/creative_eax_vs_asus_ds3d_gx_20/.