all iBooks use ATA100 (aka "IDE") 2.5" Hard Drives, which currently top out at 160GB - don't expect them to get any bigger. The SATA drives get much bigger.
I'd pick up a 7200rpm 120GB Seagate or Hitachi, you'll find a huge speed boost over the stock drive. Unless an extra 40GB is worth the drop to 5400rpm.
Hope that helps.