And if I do this I can still boot from my old HD until I get everything transferred over I guess?
I just want everyone considering adding a second drive to remember that you need to buy another flat sata cable. It costs about $35 on its own from ifixit, owc, or ebay but I recommend buying the kit from OWC for about $45 that includes the tools you will need as well as a printed manual and rubber bumpers for the drive cage.
I've been taking apart machines for nearly 30 years and I gotta say this is one of the trickiest installs ever. If you have wide fingers this job could be really frustrating.
I've done the second drive add about 7 times and once the power light didn't come on and i had to start over. One time one of the pins popped out. Bad luck indeed but I was moving too fast probably. You really need to take each step slowly and not try to jump ahead. I'm used to not even reading manuals.
That being said. The single drive change is simple as people have mentioned.