I found this very helpful tire size chart that comes in very handy when trying to figure out tire sizes for new set up.. ( just click where it says "show" on the right of the wheel size you need.)

http://ejelta.com/tiresize/index.html

This is also a handy calculator to use: http://www.willtheyfit.com/

Gives you an idea of the amount of stretch for a certain wheel tire combo, also tells you (if you know your current wheel clearance) if your new wheels will clear everything or not.