Depends on the bike! Mine isn't worth upgrading anything on. I'd do much better just buying a whole bike.
http://www.thechronicle.com.au/story...-assault-woma/
I was one of the two cyclists. Had seen him earlier in the distance doing a burnout round a corner. Was heading up the street when my mate and I heard something coming and looked round then got up onto the footpath quick smart. Rear left tyre was gone and sparks were flying from the rim on the road. Idiot turned down the street right in front of us and right past a cop car sitting on the left of the road and then round the corner when we heard more noise, obviously him crashing his car. made for an interesting ride!