Well I can't say for sure but I got a Xonar DS and it packed a punch. Bass is deeper and more "natural, the highs are more detailed, the mids are also clearer and the overall quality is better than my onboard soundcard from a regular Motherboard. For the headsets I can't say for sure but overall a Xonar DX + analog headset (hehe analog > usb headsets) I'd say I'd go for the second combo. Be sure to check for Xonar DX driver issues before though, I remember the DS had some issues with drivers from about half a year ago (can't really remember what they were, but some of my games would CTD) although those were fixed short after.
I decided to just go ahead and buy the Xonar DG and Corsair HS1A, because the rebate on the Xonar DG is almost over, and the HS1A's are basically discontinued now, and I didn't want to lose out on the discounted price if they sold out.
I can always return them or sell them back before opening, if I decide to go another route before I receive them in 2-3 days.
If you haven't made your purchase yet, I would recommend the Xonar DG over the DX. The DG is a PCI socket with a headphone amp. The boost from the amp does wonders for my HS1A. The lower Db out and the fact that it is a 5.1 card don't seem to out weigh the amp.
The only thing I've second guessed myself on since ordering is whether I should have gone with the Razer Carcharias instead of the Corsair HS1A. However, based on what I've seen online, there's no clear choice for everyone. Some prefer one and some prefer the other. So, I am just going to stick with the HS1A unless I try them out and really dislike them.