So TEG. You're telling me you'd sooner buy a drive from Apple, pay more for a small drive just so you get AppleCare cover on the HD? Where as the average OEM/Retail drive comes with 5 years warranty and costs less... Yeah I know.. Buying one from Apple doesn't void the system warranty.... but installing more HD's on the MP is user serviceable, just like installing RAM!
The MP's user manual (which everyone has) gives you step by step instructions on how to install more drives, on the current chassis it's completely idiot proof. One would have to be a complete f#$kin moron to screw up! Seriously.. those people who can managed to mess-up and kill their system/hardware don't deserve to be using them...
Actually, NO, I would do it myself. I'm only saying that if someone is not comfortable with computers, they should get Apple do it. Plus, you can buy the components you want, wherever you want and just get the Apple Store or Authorized Repair center to do the install. I've done things with my systems that Apple doesn't even allow to be done in the stores, and I'm a Computer Engineer, I know and understand how to do the work. My only concern for the OP was if they were unsure about what kind of hard drives you need for a Mac Pro, it is likely that they shouldn't be messing with system themselves, because if anyone were to read the manual, they would find that the exact the specs for hard drives to go into the Hard Drive bays is printed, plain as day, within.
TEG