fsync, shared_buffers and sort_mem

I'm trying to do some performance tuning here and wanted to know a few
things:

- Is turning fsync off a good idea? What is the worst case that can happen
when it's turned off -- if PostgreSQL crashes does it mean just data
loss or can it corrupt the db's somehow too?

- How much should I leave shared memory for other apps than PostgreSQL?

- How does sort_mem affect performance?

Regards,
Peter