So I wanted to replace my stock 10gig drive withe old 40gig from my Windows laptop (Which I upgraded to 100gig).
Step 1 - Take it apart

Eventually you'll reach the drivey goodness inside.

Success! Now take the old drive out (this was a bit fiddly I remember)

Step 2 - Put the new one in and put it back together. Now put the new drive in and start putting it back together, just enough so that yu can turn it on and use it because if there's somthing wrong then you'll have to take it all apart again.
Speaking of something wrong...
Step 3 - Use it.
I went to install OSX and it didn't find any hard drives.

So I booted up in target disk mode and connected it to my Mac Mini by firewire and worked some magic. (I can't remember what I did here, I think I just deleted all the files on it.

Back on the iBook, I tried again and it sprang to life.

I formatted it using the disk utility, and it was good to go.

Job done. Don't ask me for details because I don't remember and I dont have the iBook anymore.