well if you are having buffer underruns from IDE CD Burning, then USB will be worse.
normally you shouldn't really have much else running besides the burning program (I'm on a K6-2 400, with a slower hard drive), and perhaps your MP3 player (it doesn't take much CPU time!).
I personally, would rather have everything in my computer SCSI, just becuase it's one more step towards relieving the CPU from doing extra stuff, and therefor I get more speed out of my computer.
if you saw that comparison at toms hardware between Firewire and USB and IDE Hard drives, you'll notice that USB is horrid for Hard drives. not only do they (currently) have horrible throughput (12 megaBITS/second I think), but they eat CPU time worse then IDE. in fact, becuase Firewire eats less CPU time, it actually was faster then IDE in some circumstances!
your CD Burner will not be able to hit anything faster then 1.5 megs/second if that above number for USB is correct.