having a 2nd HDD will not diminish the performance of the first.
i guess my answer via PM the other day wasn't sufficient
1) page files/scratch disks (for programs that use them like Photoshop, even Recode) on a separate disk than your OS. having your "working" files on a separate HDD helps minimize fragmentation and is faster.
2) manipulating files between 2 drives is faster than on 1 drive. copying, editing, etc.
3) backing up.
4) IMO, you can NEVER have too much HDD space.