I used this guide to replace my iBook 12" hard drive and it was excellent. Get well prepared, print out the screw guide page to place the screws & bits on as you remove them (I stuck them down with blue tack just in case) and as wm says, take your time.
You'll have a great feeling of satisfaction when it's done (I did, anyway), and if you put a larger drive in you'll get a good price for your machine if/when you sell - I just sold my 900Mhz for £350 on ebay & I'm sure it was cause of the 80G drive.