Don't run PG without fsynch. PG 7.2 greatly improves insert/update performance due to fsynch'd write-ahead logging and periodic checkpointing rather than constant data fsynching. The disks should finish writing their cached data in the event of a software crash (of the RDBMS or OS), so you are protected in this case.
However, you're vulnerable to power failures.
Ideally you get around this problem by having enough battery to allow all writes to finish before your system shuts itself down when told to by your smart UPS. The one that never fails and the one whose battery you remember to check every once in a while, even though that cheap, low-end local ISP colo service you're using doesn't provide 24/7 access and is in a very bad neighborhood.
That gets you around the power failure problem.
It doesn't save you from hardware problems. This isn't terribly important in the sense that you can't protect against hardware problems in any absolute sense, but can only try to remove single points of failure and hope that redundancy keeps problems away. Mirroring with separate controllers for each side, for instance.
