First, the 840 Pros are truly drool-worthy. I have 2 of them and I have been VERY pleased with their peerformance.
That being said, yes, you can use them as external storage devices. The 840 Pros also (depending on the bundle) ship with cloning software (like Ghost) to help you move your OS from your HDD to your SSD.
A clean install is the best way to go. I did that with one system. On the other, I actually used Clonezilla (just google it) to move a fresh HDD install to my SSD. The SSD controller handles all of the filesystem differences. Performance on the second system is just as awesome as the cleanly installed system.