You will want to stay with the same (or close to the same) circumference of tire. You can use this Tire Size Calculator
to find suitable tires. The stock 17" tire is 235/55R17, but the stock 16" tire is 225/60R16. The circumference is 85.4in and 83.7in respectively and that would throw off your speedo (if you have stock 16" rims/tires and go to 17" 235/55R17) by 2.1% under so if your speedo says your going 60mph, you are actually going 61.2mph.
So long as you stay somewhere close to the factory circumference, you shouldn't have to worry too much about your speedo being off.
A good match to your 225/60R16 in 18" rims is the 245/45R18. This is practically an exact match to the circumference of the original tire. It also has a much wider contact patch for better traction in the dry.