the problem with scaling out to multi cores with a focus on ram is that in larger datasets you end up trading disk latency for network and protocol latency. I a not sure that is a great trade even if we are talking about fiber channel as a medium.
Cassandra and HBase Shard, they just shard differently since the shard is baked into the system while in MySQL its not. If you get right down do it, When you Shard on Toku, it will simply be more efficient at a storage level.