You'll need to find out your stock offset and wheel width, then going off the new wheel width determine how far out you want to move.
Example with completely made up numbers:
Stock 18x7 +42mm offset
Assume you have 20mm you want to move out
New wheels 17x7 +22mm
If the new wheels are wider you will need to figure half of that width into your figures.
If the new one is 17x8 that's an extra 25.4mm overall, so 12.7mm further out without changing the offset. To get the same appearance you'd need a wheel that's 17x8 +34mm offset.
None of this takes into account the tire width, those are measured at the tread not the sidewall. And they're all measured at different spots of the tread shoulder so one tire might fit and another might be shaving. Just something to keep in mind.
http://www.discounttiredirect.com/direct/infoWheelOffsets.do