oh...this is a nice Shoutcast memory for me. I had very similar experiences when I first got into shoutcast. I'm guessing there just might be a previous startup of the shoutcast server running...you might not have shut it down correctly from a previous start it up. Try this command; ps -U (your login) and see if there is another shoutcast server running. If there is, write down the pid...kill it...then try it again. If not...you have something using that 8000 port. You can change to a different port number in your shoutcast config file. This is how many peeps run several different shoutcast servers for different radio shows at once...they have different port numbers assigned to a shoutcast server within one box...and they have to be in pairs for shoutcast..ie., ports 8010 & 8011 and 8020 & 8021, etc.
...on a side note...500 users?!!...I hope you have gobs of bandwidth and if you're streaming at 128, I hope your last name is Gates.