Originally Posted by Doahh
Everyone else has said that I could use my current cassete and derailleur as long as I don't shift into gear 9
No that is incorrect. The cogs on a 8 speed cassete are spaced further apart then on a 9 speed cassete. Therefor an 8 speed shifter will pull more cable then a 9 speed.
You could easily fix this problem by buying a 9 speed cassete and a 9 speed chain, chances are that your current derailleur is compatible with both 8 and 9 speeds.