It's not Leopard that's limited, it's the chipset itself. So if the motherboard supports more than 4GB of memory, then Leopard will access it just fine.
Leopard is a 64-bit OS, so it can handle somewhere in the terabytes of RAM
From what I've gathered the current MacPro supports upto 16GB, but it all depends on number of DIMM slots on the motherboard.
Some Intel Server Boards support upto 32GB
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