A terabyte of RAM is pretty cheap. Around $12K for the RAM. Last I quoted out a system for VoltDB, the total cost (complete servers with CPU, disk, RAM) came to ~$17/GB to $22/GB.

If you actually have transaction processing at this scale and need that performance, the RAM cost is not a major issue.

Well, 2-way Cisco servers can hold 1TB RAM each.

It scales as long as throughput increases while new nodes are added. I've done benchmarking up to 12 nodes, and it continued to scale nearly linearly. (http://www.infinisql.org/blog). I'd like to push it further, but need $$$ for bigger benchmark environments.

Badly. But scaling in dataset size, and scaling in performance are not the same thing. Busy eshop might need no more than 5 GB of space (growing 100 MB per month or something) but require very high speed.