I'm guessing you're talking about the iPod Classics. I may be mistaken but as far as I can remember the big differences were:
- A slightly new look (matte black and silver replacing glossy black and white)
- A significant bump in hard drive capacity (the bottom of the range iPod classic jumped from 30GB to 80GB)
- New firmware and a snazzier GUI (with extra features such as Cover Flow)
Someone else here probably knows more!