9 speed has closer gear ratios, that's the main benefit. It's definately worth it. Keep the front der if it works. I run the long XTR rear, it is sweet but I really don't know the advantages of the long over the short.
If you are running 8 speed now, you will at least need a new chain, new cassette, new shifters, and I would check and make sure your chainrings will work with a 9 speed chain. Most will, but not all.
Look into the SRAM stuff too, you can find deals on package components to turn your ride into a 9 speed. I have one bike with SRAM and it works well, not as good as the XTR, but still very good...