True story: a friend a datamining company replaced their cluster with a single machine loaded with RAM.

Result: ~100-1000x speed up over their cluster and for less money $.

Lesson: Look at real numbers and choose your architecture to suit your specific problem. Don't underestimate the speed of RAM.

It really pains me seeing the extent to which people equate what MySQL can do with what RDBMSs in general can do. MySQL's sharding is well known to be an abomination.