importance of fast disks with pg

Hi

During the somes I did I noticed that it does not necessarily seem to be
true that one needs the fastest disks to have a pg system that is fast.

It seems to me that its more important to:
- choose the correct methods to use for the operation
- tune the pg memory settings
- tune/disable pg xlog/wal etc

It also seems to me that fast disks are more important for db systems of
the OLTP type applications with real concurrency of both readers and
writes across many, possibly larger, tables etc.

Are the above statements close to having any truth in them?

regards

thomas