According to this doc the maximum possible number of users on 4006 is 240 and 6513 is 576, are you using shared media??
http://www.cisco.com/warp/public/cc/pd/wr2k/urto/prodlit/urtrt_wp.htm
Depending upon the supervisor enginee you are using the CPU will differ
Cisco Catalyst Supervisor Engine III
CPU: 300 MHz
Cisco Catalyst Supervisor Engine II
CPU: 150 MHz
Can any one shed some light on the maximum possible DHCP pools??