paulboy...should be no problem to add the drive.
Most drives are shipped as "master" not slave. so you will need to change the jumpers. most drives have the jumper settings right on top of the drive-right on the label. if the jumpers settings are not there (or illegible) you can usually go the the drive makers site and find it. just do a "search" and type in the drive model number.
Once you have the drive set to "slave" or as they say "terminated" properly, you are ready to install it.
open the mac case and look for the other drive. where the grey ribbon connector connects to the original drive, you should notice another black connector just like the one connected the drive. If you don't you will need the ide cable with the additional connector, however i believe you should already have it.
remove the grey ribbon cable, and the power cable (it has four wires and a white plastic connector-you should notice an additional power connector there as well-just waiting for you new drive) the power connector will be difficult to remove for me at least they always are a pain to get out.
After you have removed the cables there will be a single screw in the center of the carrier the drive sits on. (This would not be seen with the black ribbon connector installed because it kind of hides it) Unscrew the screw and lift up gently about a quarter of an inch from the end the screw was securing and the drive carrier should slide out towards the direction of the screw you just removed.
Once you have the carrier out you should notice that the carrier has rails above the orginal drive that you can attach the second drive to. Make sure you have the four drive screws. You can find them at Fry's or a computer supply store-they may even be included with the drive you purchase. Once you have the drive screwed to the carrier, place the carrier back in the computer case and put the screw back in place to secure the carrier. Don't do the ribbon cable because you will not be able to put the carrier lock down screw in.
After the carrier is re-inserted and screwed down, connect all cables. The slave should be connected to the center ribbon connector and the master should be connected to the end ribbon connector. Take note that the black connectors that are on the ribbon cable have a raised "pin" on the top to insure the black connector goes in properly. Before inserting the connector identify the pins so you don't do any damage.
Close the case, boot the mac and use drive setup to initialize the drive. If this is a mystery to you then just use a Mac installation cd. It should give you the option of formatting the drive before you install the OS. I like installing (but you don't have to) the OS on a second drive along with a disk utility like techtool or diskwarriror. That way if you have problems you can just boot from the other drive.
As another persons post indicated, you can find alot of information at:
http://www.xlr8yourmac.com/
There are lots of screen shots and you may even find a walk-thru of your project.
Installing a second drive is a breeze and you should not fear it. I remember the first time I did it. It was very scary. It is just as easy as installing ram. Just remember to watch out for static electricity and wear a wrist strap so you don't fry something accidentally- Although I haven't worn a wrist strap in years. You should definitly do it yourself. You will learn something and feel a lot of pride knowing you "did it yourself".
Hope this helps...
John