best arrangement of 3 disks for (insert) performance

Hi all,
I have some new hardware on the way and would like some advice on how to get
the most out of it..

its a dual xeon 2.4, 4gb ram and 3x identical 15k rpm scsi disks

should i mirror 2 of the disks for postgres data, and use the 3rd disk for the
o/s and the pg logs or raid5 the 3 disks or even stripe 2 disks for pg and
use the 3rd for o/s,logs,backups ?

the machine will be dealing with lots of inserts, basically as many as we can
throw at it

thanks,
Richard