The oldest part of my gaming PC is the sound system comprising of a 6 year old Soundblaster Audigy and a set of Creative quadrophonic speakers.
3D sound with environmental audio effects used be a big deal. I can still remember the first time while playing an FPS that I heard footsteps behind me and spun around to answer my would be assailant with a shotgun blast. In terms of sheer immersion I can honestly say that high quality sound has always sucked me in far more more than high quality graphics.
Yet sound cards and 3D sound seem to have become the forgotten side of computer gaming. Many modern games don't support environmental audio or 3D at all. Those that do often offer half hearted support - the rear channels play ambient musak. Even games that make an effort to support 3D sound sometimes get it wrong. In Bioshock for example there were some shockingly bad examples of positional audio. Certain sound effects actually had a dead spot - they would disappear altogether as you spun around.
On the hardware front sound cards seem to have become moribund. Although Creative and other manufacturers continue to bring out ever Higher Fi versions of their cards game reviewers and hardware sites these days almost universally recommend using the on-board sound from your PC motherboard.
I have been through three different motherboards and processors since I first got my Audigy and each time I have tried out the on-board sound to see how it compared. In every case the answer is that it did not. The atmosphere, depth and liveliness I get from my Audigy is just not there with on board sound. It reminds me of the comparison between a camera phone and a proper digital camera. The number of megapixels may be the same but the picture quality is simply not present in a camera phone.
I have thought about upgrading my Audigy on several occasions but in the absence of better support for 3D environmental audio from modern games there doesn't seem to be much point. I must also confess to a reticence about upsetting my current sound set-up. It took me several years to get it working stably. While Creative have been the leaders in gaming sound for some time their drivers have often been awful. For a while whenever I had a compatibility problem with a game I instinctively looked first at my sound card. Thankfully I finally hit on a stable working diver configuration a few years back and I am loath to change it again.