I have 3 Mac Minis that I as a software update servers ... one for each of my schools. They are older and had small, 120G hard drives. With the releass of 10.6 & 10.7, these hard drives became too small to hold all of the necessary updates for all of the various OS versions of our schools' computers.
Instead of buying expensive, new servers, I replaced the HDs in all 3 servers.
HD replacements were very easy using the guides.
The first one took about an hour.
The next two were much faster.
The guide I used was complete and easy to follow.
Read all the steps and take your time.
Be sure you have the right tools ... all sold right here at iFixIt