Take a look in the JK part and do a search, tons of info. Here's the basics though as I understand it.
With a stock suspension you can fit 33" tires, but you will either need spacers behind your stock wheels, or wheels with less back spacing.
The back spacing will depend on the size wheel you want. If you stay with 17" or 16" I think 4.5" or so will work, but if you go to a 15" wheel you'll need 3.75. That's just what I think I remember reading, you'll want to double check that before buying anything!