Notebook drive question

First of all I have never messed replacing hardware in laptops before (this is for a friend at work)....What I am trying to accomplish is replacing a bad notebook drive with a new one....sounds simple to me because I do it on desktops all the time but anyways...let me tell you system specs..

Gateway laptop (all i know), p3 500mhz, 10gb harddrive (do not know the vendor)...so im guessing its a couple years old

before I go tearing at this lil project I wanted to get some input from you guys on what drives will work in a laptop and maybe make a couple suggestions....I didnt know if there were different size drives, and if some would work or some wont etc...The notebook drives I have seen are 2.5" drives 9.5mm do these fit into all notebooks??.I really want to know what I am getting into before I get my feet wet...I appreciate your input!...I popped the keyboard off yesterday and I think this is how I get to it so I have a start =]

1. Measure the height of the dead drive.
2. Go out and buy a replacement laptop drive that is of the same height.

There are only a few heights used, i believe 9.5mm is the comon size. 12.5mm are for larget capacity lappies.

<b><i>The Very Hungry Caterpillar</i> - George W. Bush's favorite childhood book.
Note: This book was first published a year after Mr Bush graduated from College.</b>

Thx guys