I'm looking to put on some new wheels.  What's the rule of thumb for fitting different tire widths on wheels.  So on a 15x7 wheel, whats the min and max tire w/o body mods?

Can you do it? Yes, but should you do it? NO. The 205 shouldn't go on a rim any larger than 6.5" wide but a 215 can go on a 7". For a 7" rim you can lok at tire widths from 215 to 255/10.5" wide.

As for min and max without body mods, well you also need to consider suspension lifts, bodylifts, wheel offset, width of tire and height of tire. What's your setup and what do you want to try and run?