For sound quality the Audigy isn't the best sound card
If you don't play certain games and sound quality is more important, look at Lynx, RME, M-Audio Delta series, or Creative's other line of sound cards: Emu 0404, 1212m, 1812m. These cards are designed for musicians and home-studio hobbyists, but some of them are affordable--the Emu 0404 at $99 is cheaper than some Audigy packages.
I have an Audigy 2 and an Emu 0404 (also other recording cards with balanced inputs), and for me, the 0404 and the other recording cards sound noticeably better than the Audigy. This has been confirmed by review sites that measure sound card performance. To name one random example, Tom's Hardware Guide. Look up the Audigy and the 0404 on THG and compare their measured audio performance.
But why speculate about the Zen's performance? Measure it! Download RightMark Audio Analyzer (freeware), put the calibration and test .wav files into the Zen, and then use your sound card to measure the Zen's performance. I did the same with my 1st Gen iPod Mini and an old Yamaha DS2416. The loopback performance of the DS2416 is much better than the iPod mini's, so the numbers for the iPod aren't below the measurement limit:
1G IPod Mini as a line-out device (i.e., with no load),
volume level max:
Frequency response: almost ruler flat 20 Hz - 20 kHz.
Noise level dBA: -90.5
Dynamic Range, dBA: 90.3
Stereo crosstalk, dB: -89.7
IMD (swept freq.): 0.0094%
As a line-out device, the IPod mini has pretty good numbers. Now here's the problem with the iPod and the other portable players: the performance gets a lot worse when it's driving a pair of earphones. Here are the numbers when it's driving a pair of Sony MDR-EX81SLs:
Frequency response: compared to 0 dB at 1kHz:
-0.5 db at 20 kHz
-3 dB at 70 Hz
-4 dB at 60 Hz
-5 dB at 50 Hz
-6.5 db at 40 Hz
-8.5 db at 30 Hz
-12 db at 20 Hz.
It's too bad we can't post graphs and charts on CNet. The fundamental frequency of the lowest note on a piano is around 27.5 Hz. Pipe organ goes much lower. So if you use a normal pair of headphones on the iPod, you're going to get really wimpy bass performance. The bass boost EQ preset is really mild and won't make these numbers go up. Other MP3 players have the same problem, but I've heard that the problem has been fixed with the iPod Shuffle (maybe somebody with an iPod Shuffle can make some measurements).
The solution is a portable headphone amp (bulky), or a pair of earphones that have a hyped-up bass to compensate for the iPod's bass performance. The Sony MDR-EX81 that I use is such a pair--it sounds balanced with an iPod, but sounds too bassy with a headphone amp. Sony has a couple of other 'phones that do this: MDR-G74SL, -51LP, -71SL, -NX1, etc. Those will probably sound horrible on a regular stereo (unless you're a basshead).
Even when driving the EX81s, the iPod Mini has somewhat worse numbers on the other performance parameters:
Noise level dBA: -90.3 (breathing is louder than this)
Dynamic Range, dBA: 90.2
Stereo crosstalk, dB: -48.7
IMD (swept freq.): 0.844%
But it sound fine to me through the EX81s. Unless you can make a side-by-side comparison of two MP3 players, it's hard to compare their sound quality. I compared my iPod Mini to my MPIO FY400 flash player and my EMU 0404 going to a headphone amp. The iPod Mini has the wimpiest bass of the three. That's how I know about the sound quality even before taking measurements. Let your ears be the judge.