There is a table in the master DB called syslogins. Within that table there are integer fields: (sysadmin, securityadmin, serveradmin, setupadmin, processadmin, diskadmin, dbcreator). If a user is a member of one or more of these roles, he/she will have a '1' in that field. Otherwise the field will be populated with a '0'.
If you use the syslogins table along with the sysusers table in your user DB(s), you should be able to retrieve any data you need to...
Craig