100GB is pretty large.. I have a server with MySQL/Innodb and about 30GB of database space. Dual P3 1.26Ghz CPUs, 4GB RAM, fast 10KRPM IBM SCSI drives. Excellent results. I have no experience with the other databases you mentioned but can recommend Innodb over MyISAM. Read about it at MySQL's site or at
www.innodb.com Good stuff.