cgilmer has a very good suggestion. A new server or more hard drives is fairly inexpensive and very worth the cost during a symposium migration.
I will be upgrading from 4.2 this week to 5.0 with database replicator, hot and warm servers. I went out and bought a few really nice dell 1750's that I will be migrating too. If for some reason the migration doesnt go well, I plug back in the old server and reboot symposium. Im back to where i started.
Keep your rollback options completely open and easy.
Hope that helps,
Dusty