Hi,
The Radvision Pathfinder is a firewall and will probably be your connection to the internet. The Cisco 6506 is most certainly your core switch and will connect all of your devices. The 3825 is has a lot of functionality but is probably set up to be a voice gateway. This makes it your connection to the public phone system. The IPS 4240 is an Intrusion Prevention System and will sit between your firewall and your core switch. MARS is an analysis tool that works with all of your devices and puts your syslog messages into something easy for humans to read. The 7825 is essentially an IP PBX and will be the brains behind your phone system. The VTC 3545 is a video conferencing server. This will control your video conferencing.
This is certainly an off the cuff answer. I would need a lot more information to give you a complete design. There is some pretty advanced stuff in your list of equipment. I would be glad to talk to you about it.
You can reach me during west coast working hours at 800 571 1717. Ask for Brian Bayer.
Brian ": )