Preface: I'm a moderate audiophile and many people might not share my views or be as picky as I am. I have never really liked any gaming headphones all that much. I own the following headphones and equipment:
Sennheiser PC350
Sennheiser PC360 (my main PC headphones...I EQ these to tone down the upper bass...and I use these over my other headphones because I need a good mic)
AKG K702 (my music headphones)
Ultrasone HFI-580 (my 'fun' headphones)
Source:
Realtek 889
Astro Mixamp
Maverick Audio D1 (only use it when listening to music...don't really prefer it to my Realtek 889, so I wouldn't waste your money)
I've also tried the Xonar STX, but it wasn't that big of an upgrade over the Realtek 889. As the Realtek 889 is the highest onboard sound they offer, it's actually not half bad, so I see no reason to replace it.
---
I don't know if there are better alternatives if you really need Dolby Headphone, but the Astro Mixamp is not that good if you're not bothering to use it for its mixing capabilities. It has serious shielding issues, and is a pretty poor amp. My PC onboard (Realtek 889) sound is a fair bit better.
I still use the mixamp when I want to use headphones when gaming on consoles, though.
edit: woops, I see you ordered it. No big deal...it has its uses for consoles especially, as it will make using your headphones with it a breeze. On the PC side of things, just use the Xonar DG. If you need a bit of extra bass, don't be afraid to EQ your Xonar a little bit. Give a bit of a boost in the 20-100Hz range, or even up to 200-300Hz if you want a bit of mid bass. You won't need to touch it much, just a few dBs at most to get that oomph back.
Then again, your perception of bass might be different than most. I have the AKG K702, which many people say lack bass, but I find they have great bass, so that's that.
If you ever do go to get another set and want closed headphones that sound incredible for gaming, be sure to check out the Ultrasone HFI-580.