postfix+dovecot howto for an small mail server (router too)

our aging rh9 mail server started to show it's caps age and started to fail (p3@350Mhz with 256M ram and 2G hdd)
on each day (hw errors) so we are switching to an more modern ubuntu gutsy on an
p4@1.6Ghz (712M ram) and 40G WD drive

Old config was sendmail+pop3+shorewall
The new config is postfix+dovecot+shorewall
Here is the guides i followed
https://help.ubuntu.com/community/Postfix
https://help.ubuntu.com/community/Dovecot
http://www.shorewall.net/two-interface.htm

ps: some of the hw errors we had in the logs

WARNING:  Kernel Errors Present
  hda: task_no_data_intr: error=0x04 { DriveStat...:  2Time(s)
  hda: task_no_data_intr: status=0x51 { DriveReady SeekComplete Error }...:  2Time(s)