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.
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.
