While I built my system about 6 years ago, one of the expensive parts I had dropped cash into was my sound card. I wanted digital outputs, and a card that supported DDL and DTS encoding. The card of choice was the Auzentech X-Plosion 7.1 DTS Connect. It was the most high-end card of it's time, but shortly after (about 6 months later), the X-Meridian 7.1 was released, which has a better DAC. The DACs on these cards are upgradable, which was another selling point for me as far as flexibility with my options went, more bang for the buck.
I did have an X-Mystique 7.1 Gold before buying the X-Plosion (the X-Mystique was released at the same time), but the DAC is of lower quality, and I did notice a difference between the old X-Mystique and the better X-Plosion, even on the Logitech Z-680 5.1 surround sound system I bought to go with it at the time, which supports DDL and DTS over digital optical and/or digital coax (no DDL or DTS over the analog inputs).
The software for the cards is nice, but it was buggy upon first release, I had issues with it freezing audio playback if switching between sources during playback. The issues seem to have been fixed since.
One drawback to using the older Auzentech cards is that the surround sound in older games (Doom 3 engine based games) is not supported in any version of windows upwards of XP varients. I am still running Windows XP Pro (no need to upgrade, really), primarily due to the fact that my surround sound would not be supported while continuing the use of my current sound card.
Prior to using Auzentech sound cards, I was a huge Creative fan. They have been around for ages, and I still use their oldest and original Sound Blaster ISA slot card in my DOS computer. Their X-Fi was release at the time the X-Mystique was but lacked the DDL and DTS encoding, so surround sound wasn't an option back in 2006. I have experienced the "quality" of the original and second generation X-Fi cards, and I can personally say that the sound seems much more muffled and/or flat compared the Auzentech cards. The use of the software based mixer is mandatory to get a decent "clean" sound. I do not like to use software mixers my self, so Creative cards were out of the question at the time. I cannot comment on anything new that they have release since 2007 though. I would assume that the quality of the hardware has increase, but whether or not it is on par with what AuzenTech has to offer, I cannot say.
If I were to upgrade my sound card now, I would continue my loyalty to AuzenTech and go with their newest X-Fi Home Theater HD Card, currently running for $249.99 on their website, due to the amount of quality in both the delivered audio their cards have proven to put out, as well as how much more pleasing the hardware is to look at (I'm a hardware buff).
If you are bigger into the sound effects of games, then Creative would be your way to go. Not sure if they still do or not, but Creative owns(ed) the rights to EAX, which is the effect "engine" of most games. High end creative cards would support the latest and greatest EAX versions before Auzentech or Turtle Beach cards would. With my X-Plosion and the old X-Mystique, they supported a max of EAX 2.0, while the latest was 4.0 (which my previous Audigy 2 ZS did support).
Another note to take into account would be that Turtle Beach is another sound card manufacturer that has been around for awhile as well (since the mid 90s), and I am fond of their cards as well. Their high end sound cards are very similar to AuzenTech's cards in terms of sound quality, however features and software are skimpy. For example, when the X-Mystique was released, Turtle Beach release a new card shortly after that used the same DAC as the Mystique, but had less options for outputs (but their card had optical in while the Mystique did not, with the sacrifice of the coax output), and the card was $20-$30 cheap than what Auzentech was offering.
I now own a lower end component 5.1 surround sound system that I have pieced together that would take more advantage of a high end sound card than the Logitech Z-680 system would. I still use the Logitech system, however not on the PC. It's quality is amazing for a set of PC speakers and will blow any HT-in-a-box out of the water at half, and in some cases a fraction, of the cost.
Edited by PintoDave - 11/7/12 at 8:26pm