http://www.willtheyfit.com/ is a good site for checking out wheel offsets and tyre rollouts.

Just stick in your existing info and then your proposed wheel size/offset and you'll see. AS ridgey said, if your current wheels are (lets say) ET40, then you'll need 50mm less offset so ET-10. An 8J wheel with an offset 30mm less than your current wheel would stick out 49.1mm more without requiring the use of spacers.

Hope that helps.