I suspect that if you took an mp3 file encoded at 64 kbps and one encoded at 256 kbps and played both through average speakers, the average person wouldn't be able to tell the difference.
(In case you don't know what encoding and mp3 and kbps mean, basically an mp3 is a music format, encoding means to stick the music into the mp3, and the more kbps, the higher the quality of sound. Why don't we encode everything at some insane kbps level, then? More kbps makes the file bigger, so there's a tradeoff between file size and quality.)
Why is this important? Because 64 kbps is absolutely AWFUL sound quality. 256 kbps is considered very high quality. This means that even if we used high-quality encoding, etc. the average person couldn't hear it, because their speakers and computers can't recreate the sound accurately.
This is significant because most people will go through their lives having not actually listened to the music, but simply having heard the tune. While the tunes are great, there is another level of enjoyment one gets out of actually being able to hear the music.
It's also significant because most people encode their mp3's at 192 kbps, whereas they could halve that and not hear the difference.
Finally, many people not only have junk speakers, but they also listen to their music very loud. This actually distorts the sound as the speakers are driven past full excursion (that is, the cones try to move farther than they physically can). This distortion is not always bad; many guitarists and bassists use this phenomenon to make their 'signature' sound. It's bad in this case, though, because the music was recorded, produced, and mastered to sound good without distortion.
You can read about my experience with this phenomenon here.
So what do I recommend you do? Go buy a pair of AKG K44 headphones. With these, you'll actually be able to hear the differences between your iPod and your computer.
If you've got a little more money to burn =] buy yourself a Behringer UCA202, a nice small amp (or do what I did, and get a small soundboard. You can get them from Behringer for ~$30), and hunt around on Craigslist for a set of nice speakers (like Pioneer CS-G403's). With this setup you'll actually be able to hear amazing things in the music. In fact, they're so good I don't recommend you try to get anything else done while listening to them. You'll find yourself stopping to listen to the music. And you'll actually hear ALL the music.