Top tips for WMPU install

Our webhost will set up WPMU on LINUX VPS with CentOS 5/Plesk/20gb/512mb ram.

They say they haven't done it with WPMU before.

Any tips/reminders specific to WPMU installation would be much appreciated.

Also, there are three choices for databases: phpMyAdmin, MySQL 5, PostgreSQL server.
Which one is most suitable for WPMU? Should we ask for Multi-DB?

Thank you.