The sup720-3B supports 256k routes, if I was to upgrade this supervisor to 1GB of ram from 512MB, will this allow me to support 1 million routes like the sup7203bxl or is this not possible?
In short, No. Upgrading the 3B to 1gb rp/sp memory will not allow the FIB to scale to 1,000,000 routes.
The 720-3B is scalable UPTO 256k routes. This is due to the 'forwarding information base' design in this sup that scales to 256k routes only.
The 3BXL has a FIB that will scale to 1,000,000 routes of IPv4. (500,000 of ipv6)
Please see the following link for more info on the 3B:
The following link is a data sheet that describes all 720 versions: