What's the normal setup of nodes?

[Rasmus Henriksson]

I can imagine that the recommended ratio of data-nodes / mySQL-nodes,
is different for different projects. Could someone please describe the pros
and cons on these three setups, and if any of them are good for one kind
of job, but not on another one:

SETUP 1:
2 mySQL nodes, 4 data-nodes.

SETUP 2:
4 mySQL nodes, 2 data-nodes.

SETUP3:
3 mySQL nodes, 3 data-nodes.

Thanks! / Rasmus