Ditto above. For a variety of techical reasons I won't bore you with, I've swapped HDs in and out of a Ti 400 and 550 15-20 times and it is quite straightforward if you follw the links.
The only tricky bits are the little rubber washers/dampers that sit on the sides of the drives that you run the screws into. It's touch to get these sitting stably in the drive while you maneuver the flexible plastic tongue back up against the HD. You'll see what I mean when you're there. Usually they are tacky enough to stick long enough to wiggle the drive in but I've hearc of people using small amounts of rubber cement to hold them in place. IMO, that's not such a good idea since you may need to get back in later on.
Note: keep track of which screws come from which hole as sometimes they can also be fussy about going back in (Ibelieve that Apple even suggests proper "order" for them to be removed/replaced). You have to have a firm hand with the screws to keep them from stripping as well.