Sorry, just wanted to add:
I checked the "Create Portal" page - Portal Alias and Portal Home Directory are 2 different things - it looks like the "Portals" table holds the "HomeDirectory" column, so I would need to change this from "Portals/6" to "PokerDIY" I guess. The PortalID in the PortalAlias table remains the same (still numeric).
Can anyone confirm this? I think you'd have to check all modules to see if they storing the full image path and run a SQL script on any table that does...
Lastly - is there a performance impact of using an alaphanumeric field over an integer field? (something like SQL indexing etc, or does it not apply in this case)