My best information:
The SUP720-3BXL ships with two 1 GB DRAM chips. 1 is used for the route processor (BGP, OSPF, etc) and 1 is used for the switch processor (VLANS, MACs etc).
This is the maximum amount of memory it supports; it cannot be upgraded.
Some of this memory is reserved for things like TCAM, so you have about 895 MB to work with when booted into IOS 12.2.