SNMP "Accounting" Software

We need some fairly complex SNMP accounting software (data center) style stuff that can monitor cisco equipment for bandwidth utilization and generate reports based on 95th percentile and also perhaps even their actual bandwidth usage (how many gigs of transfer they use per month, day, week.. etc) Does anyone know of anything good that does anything like this? It needs to be reliable? Can be open source, we're using MRTG to track utilization but we need something that really handles "accounting" for us. Thanks, -Drew