Neo4j is limited to the ram available on a single machine. Cassandra can scale across many machines. You won't get nearly the same performance though as you would with neo4j.
Small correction: neo4j is limited to the resources of a single machine, but the database doesn't need to fit in RAM. You will see a significant slowdown once your data gets too big for RAM, but neo4j has put a fair amount of effort into efficient caching.
I've done something similar over Voldemort. Local traversals are probably comparable, depending on how they partition and how much duplication there is. And then, you get to do large traversals, which is nice.
I don't know how good Agamemnon is, but I like the approach.
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