I tried a number of external USB soundcards, including Creative Live! 24-bit external (too weak output and huge), the DIAMOND XS71U 7.1 recommended (weak output and pop on start up), and the Creative Sound Blaster X-Fi Surround 5.1 (very loud pop on start-up). The start up pop may vary depending on computer, power supply, and USB settings. The best card I have found is the Creative Audigy 2 NX. Long out of production, but still available on the net, no start up pop, and a full 2 volt output to your amps with a required 5V external power. The X-FI surround also is rated at 2 vrms, though without external power it is unlikely sustainable. Most of the other USB cards available don't come close to 2V output, meaning you would have to increase the gain on your amps and potentially introduce noise or overdrive your amps for the same sound output. The X-Fi sounds great if you can get rid of the pop. I could not, even with a range of configurations and without resorting to a start-up timer.
DACs are essentially the same thing as a soundcard with a fancy name for generally more expensive, better quality, and better sounding external sound devices that operate with native drivers. You can run the X-Fi as a DAC - just plug it in and don't install the Creative drivers. The DACs starting at $100 and up (way up in some cases) will sound better with some significant caveats. They generally only provide 2.0 or 2.1 output, which is not a problem to car stereo enthusiasts who run only front speakers and a bass (if that). Doesn't work all the time in modern, multi-speaker setups or those wanting 5.1 sound (heck, even 4.1 sound). DACs often require an external power supply which can add a level of power management issues. The biggest issue is that to get the best sound, which is the reason you pony up for an expensive DAC, you need a hard wired volume control. To get the full benefit of a DAC, windows and program volumes need to be set at 100% so they don't alter the signal to the DAC, with the DAC or and amp controlling the volume. Not so hard with a PC hooked to headphones with a small amp or your home stereo. A little more difficult in CarPC environment, but some DACs have an interface with the PC which allows control of the volume from the PC. See the AudioQuest DragonFly, which costs around $250 and gets good reviews.
Or search the internet and pick up an Audidy 2 Nx for around $25 and figure out how to get 5V from your power supply or PC.