> Just how much data do you need when these sort of clustered approaches really start to make sense?

You really need an enormous amount of data (or data processing) to justify a clustered setup. Single machines can scale up rather quite a lot.

It'll cost money, but you can order a 24x128GB ram, 24x30TB ssd system which will arrive in a few days and give you 3 TB ram, 720 TB (fast) disk. You can go bigger, but it'll be a little exotic and the ordering process might take longer.

If you need more storage/ram than around that, you need clustering. Or if the processing power you get in your single system storage isn't enough, you would need to cluster, but ~ 256 cores of cpu is enough for a lot of things.