Some call it God's country. I call it Acton, Maine
Too Many - 7 or 8
You will have to change at the very least- chain, shifters,and cassette. While I have had luck using some 8 speed derailleurs when swapping over to 9 speed, also changing the rear derailleur would insure a smoother system. The front derailleur can work with either if it is cold set properly.
The difference between 9 and 8spd isn't really anything to write home about. You can get yourself a nice 9spd derailleur and run it as an 8spd until you have money for the upgrade. Last I checked the XTR derailleurs even are still 8spd compatible.
I just converted my road bike from 8spd Sora to 9spd Tiagra.
New shifter and cogset were needed.
I tried Shimano Sora, 600, and RSX Derailers and found that they all work well with the 9spd set-up.