I was having this same problem on my Inspiron 6400 with Vista Ultimate x64.  Turns out the problem is not really with the sound driver, but on mine it had to do with my wireless card driver.  I downloaded XP drivers for my Draft N wireless card and now I occasionally get a stutter, but 95% of the time it's fine now. 

I have the same problem with my laptop.  I also have a XPS 1710 with the wireless n draft card.  Upgraded to the 64 bit edition of vista, and from then on everytime I played music or games in got some lag spikes that chopped the audio.  Like everyone else I traced it to the wireless card.  Once its disabled the sound is perfect.  But enable the wireless card and everything goes to... well it doesnt' work.  Contacted Dell support and they had no idea on how to help me.  They directed me to install new audio drivers thats it.  Which I had already done countless of times.  Eventually all they said was to wait for new wireless card drivers.  Wonder how long the wait will be.

I've disable that, the wired network, Bluetooth network and it still stutters.

There is a thread at the top of the Vista forum with the following information:

 

 

 

Sigmatel C-Major HD Audio Driver Update for Windows Vista on XPS and Dimension Systems

In our testing, we have found that some Dimension™ and XPS™ systems may need an updated audio driver in order to

enable excellent sound quality with Windows Vista. Dell advises you to play downloaded music or a movie and check

that the sound is audible. If you are having problems hearing the sound from your PC, you may need a new Sigmatel

audio driver that you can download directly from Dell. To obtain this driver, click on the following link and follow the

download and install directions below:

<ADMIN NOTE: Broken link has been removed / replaced from this post by Dell>

 

Possible workaround if audio issues are unresolved after installing the new driver:

* Click Start- Control Panel- Sound
* Right click on the Speaker/Headphones icon and choose properties
* Click on the "enhancements" tab
* Select the "Disable all enhancements" check box
* When finished, restart the PC

been there.. done that.  Still doesn't work.  The problem is not the audio drivers.  Its the wireless card drivers, or something to do with the wireless system.