under which criteria should I do this upgrade (MEM-MSFC3-1GB) for a sup720?
According to memory guidance for 6500 series;
Upgrade Considerations for memory:
(1) Large route tables >25k entries
(2) Premium features (for example, enhanced Fast Software Upgrade [eFSU], In-Service Software Upgrade [ISSU])
(3) Right size memory now for future Cisco IOS Software images on Cisco Catalyst 6500/Cisco 7600 Series Supervisor Engine 720 Cisco Catalyst 6500 Supervisor Engine 32 and ME6524
Here is more info:
This would be needed for something along the lines of "I have 512MB of RAM and I am low or out of memery when I activate all my BGP peers". Each full internet table will take very roughly 230MB. Upgrading to 1GB of memory on the MSFC will allow you to hold a bigger BGP table.
Be warned, this will NOT increase the number of routes that can be programed into TCAM for hardware forwarding. This is memory for the RP only.