Stock offset is 45mm, with a 6.5" wide rim. The stock 205/60-16 has an 8.05" section width (sidewall to sidewall) and the 235/40-18 has 9.25" section width. So you're adding 5/8" (.5 x [9.25-8.0]) to the amount it sticks out just by tire size, then you add another 10mm (which is about 3/8") and your new setup will stick out a total of about 1" more than stock. If you're lowered you will rub with peeps in the car, over bumps, hard corners (maybe) etc. You can get the fenders rolled and you might be okay. I think my math is solid here, but I welcome corrections.
If found this link really helpful when I was shopping for tires.
http://www.discounttiredirect.com/direc ... reMath.jsp
Another possible issue is that the 235 is considered by some to be too wide for a 7.5" rim. Not that it wouldn't mount, but you'll have to play with the inflation to make it sit flat and get even tire wear. I almost went for the 235/40-18s, too, but with a 45mm offset wheel. At the last minute went with 225s.