Hey there, @jon645!
@Zane is right about not loosing performance just because you have multiple hard drives in your system. However, I'd strongly recommend you do regular backups somewhere off-site as well. Always remember that "backup" means that you have your data stored in at least two (2) locations. Moving data from your system drive to an external hard drive or another internal HDD is not a backup, unless there is already a duplicate of the file on a different drive.
When it comes to drives' performance, you need to look at their specs and more specifically the RPM and cache that they have. If you really want to boost your performance in terms of boot-up and load times, you might want to consider using an SSD as a primary system...