You normally use the Catalyst 6500 to connect vlans together. Say you have a 30 story building and each floor is on its own vlan which it should. Then those vlans physically terminate on a 6500. Also called a Layer 3 switch.
The 7600 router is a true router and alhough you can do vlan routing on it its primary use is to provide a connection to remote offices. For example it could terminate a DS3 or a frame relay circuits to your remote locations.
Thus in conclusion primary role of 6500 is switching and 7600 is routing.
The cisco website probably has a better explaining and so does the CCNP books. Hope this gets you thinking in the right direction