Hey,
what application do you use to keep track of your servers, and clients and invoices etc..?
Is there some program which allows you to track the amount of bandwidth per server, as well as how much bandwidth you have sold. The amount of IPS on there etc.. as well as a client database?

try netenforcer for bandwidth monitoring
look on freshmeat to find network mapping and monitoring tools
use prism if you have the money (or get modernbill)

http://www.inovaware.com
http://www.perlbill.com
http://www.modernbill.com
http://www.freshmeat.net
http://www.sourceforge.net