Backing up a 400-500 GB database on MySQL/Amazon RDS? We have an Amazon RDS small instance for a MySQL datawarehousing database.
Database size will be around 400-500 GB very shortly.
What is the best way to backup our database?

Can it be done efficiently with automatic backups and DB snapshots available with RDS?

Or should we consider some other options? What would those be?
Using EC2, hosting in some other cloud, or using any other database?
RDS provides automated backups as a feature.  Those backups are system automated (persisting through the life of the RDS instance), or user initiated (persisting outside the life of the RDS instance).  They also automate binlog backup for point-in-time restores.  But you can't take those backups out of the RDS service, as it's all hooked together for automating restores, etc.

FAQ: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/RDSFAQ.Backup.html

I assume you want a backup outside of RDS.  mysqldump would be the only way to do it, and unless you use some creative ETL to facilitate some sort of daily delta, you might save time.  Otherwise, that's a good chunk of data to backup on a regular basis.