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QoS on a 6500 depends on many factors. For example, different 6500 line cards have different QoS features. Likely you'll be able to define a bandwidth share ratio for one of your hardware queues that will provide your desired 100 Mbps guarantee.
Much about 6500 QoS might be found here: http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND_40/QoSCampus_40.html#wp1101384