Consider the new Seagate 7200.11 drives. They are very fast and you won't need a 10,000 RPM raptor drive for Photoshop if you partition one and make the first 100 GB your Photoshop scratch disk.
There is info on 7200.11 performance here:http://www.barefeats.com/hard94.html
I have no idea how noisy the new Seagate drives are, but the 7200.10 drives I'm using are OK.
Time machine is a nice idea, but it isn't a backup solution. Make bootable backups on a regular schedule (e.g. once a week) using a utility like SuperDuper, and store them off-site. There isn't much point in having Time Machine on an external drive with a Mac Pro as you have lots of room for internal drives.