First, I would suggest using the included ipfw firewall (not enabled by default). The included “standard” Leopard firewall, although patched, exhibits some anomalous and disconcerting behavior.
Make sure you run ALL updates before putting the box into production, as there have been quite a few patches/fixes/updates within the past few months… (but you already know this).
If possible, perform a clean install: there are changes to core OS files which can at times make an upgraded version of the OS rather less than totally stable. (see here for additional information: http://theappleblog.com/2007/10/23/how-to-upgrade-to-leopard/)
As with any migration / upgrade, make sure you BACK UP your current docs / settings / favorites / etc…, especially for any major programs like the CS3 / CS2 suites.
Oh, and did I mention that you should BACK UP your system, in case the migration fails (which has been known to happen on occasion, rather more often with Leopard than with Tiger and previous…).
And don’t forget to back up……..