Virtual Memory runs too slowly. If your game play abruptly slows for no apparent reason, this is usually why. You can also turn the ambient music off entirely. Do not turn off the other ambient sounds as those are frequently hints that somebody's sneaking up behind you or otherwise about to do you in. Go ahead and turn the sound quality down though.
Most people also turn off weapons flash as it tends to block the view.
Additional information on making UT run acceptably on older machines is covered in the osX clan help pages. If your 56K modem isn't quite as speedy as you would like, perhaps you can enhance your set up. Check your modem's speed at DSL Speed Test , Leslie's Modem Speed Test Page or 2 Wire's Bandwith Meter . The first speed test link is the most useful as it shows upload speeds as well as download speeds. The upload speed is most important for a good ping.