The SB Live! outputs at 16-bit. If you use an optical out to go to a receiver, you bypass the DAC on the soundcard, and will use the one in your receiver which will MORE THAN LIKELY be better, but not necessarily.
There are a few soundcards out there, such as the Delta 2496, which can allow for 24-bit output, and will improve the quality of your music somewhat, but as Workin' said, you're going to be a bit limited by the source. Unless you're making your own mp3's with a LAME encoder at proper bitrates, you won't really be utilizing much benefit. Also, to decode at 24-bit, not only will you need a soundcard which can do it, but you'll need something such as the
MAD Plugin for Winamp as it can do up to 32-bit streams. Make sure you read the page, if you plan to decode at this bitrate, on how to properly set this up.
Also, the site appears to be down right now, not sure why but it should be back up soon.
Almost forgot to mention, if you are interested in how you can make your own cd-quality mp3's (i'm not wanting to debate this right now but suffice it to say, it's damn close, close enough you won't notice), make sure you read
The r3mix Files for some great information on where to get your tools, which ones to use, and how to set them up.
dm