master virus showed you how pretty much, also you can do neat effects with colors moving and other things like that...
but neways here are the common speeds:
25 - very fast things that don't change much each time but change a lot... (40 shows per sec)
50 - fast movement, changing slowly is best, using lots of color changes looks good (20 shows per sec, combine with 100 to make cool speed up/slow down)
100 - slow movement, good for text that appears/re-arranges itself (10 shows per sec, mix with 75/125 for cool changins speeds)
150 - good for text that looks like its appearing:
h
he
hel
hell
hello
using a flashing "_" or "I" also looks good on the end of the text.
250 - very slow, only for text messages that change very slowly. (4 shows per second)