For basic stat:
You need to activate the SMNP in the SonicWall and use a tool like PRTG/MRTG. I does not know if cacti can. (I never used that one)
Please follow one of those KB to activate the SMNP. (depending on your OS version)
Configuring SNMP in SonicOS Standard and Enhanced (SW5096)
Configuring SNMPv3 in SonicOS (5.9 & above) and (6.1 & above) (SW11183)
The MIB can be a problem, so you may have to update it too. Like this KB show.
SNMP Query for Bandwidth Statistics Fails (SW3038)
For advanced stat:
From sonicwall you can buy the viewpoint feature.
ViewPoint Upgrade
Upgrade Required. SonicWALL ViewPoint is a software solution that
creates dynamic, Web-based reports of network activity. SonicWALL
ViewPoint generates both real-time and historical reports to provide a
complete view of all activity through your SonicWALL Internet Security
Appliance. With SonicWALL ViewPoint, you are able to monitor network
access, enhance network security and anticipate future bandwidth
needs. SonicWALL ViewPoint •Displays bandwidth use by IP address and
service. •Identifies inappropriate Web use. •Presents detailed
reports of attacks. •Collects and aggregates system and network
errors.
For advanced stat, but only for a short time:
Please run wireshark with a port mirror from your lan cable of the sonicwall.