A quick checklist... I may have missed something though:
Get the crossover network configured. My last attempt at this resulted in the ethernet driver hanging (on egret fortunately, not on traven).
Ensure the external network interface is sensibly firewalled.
Optionally install a vserver kernel and set up a vserver within which to run postgres. Personally I'm not sure that we gain a lot from this added complexity.
Set up an encrypted partition for the database.
Install postgres, configured as on traven but with network access via (only) the crossover interface.
One at a time, take Mir sites down, dump their databases and restore them into postgres on egret, modify the database options in config.properties to point at egret and restart them.
Move the nightly db dump cronjob(s) onto egret.
I think that's basically it.
egret.indymedia.org.uk, in the rack at riseup, with a gigabit crossover to traven.
It's running Debian (sarge), and already has postgres listening on the crossover interface. It hasn't had any particular tuning done, but then neither really has the postgres on traven (as none of us are experienced postgres admins...)