That would be a 25.71" overall diameter... the stock 16" size is ~24.87" and the stock 17" size is ~24.61" so that's a big increase over stock. I wouldn't do it. Use the formula at Tirerack.com in the link below to figure out the approximate overall diameter tire size. If they are 18x7.5 wheels then you should probably go with 225/40/18's and if they are 18x8 wheels then you can stick with the 225's or move up to 235's because of the extra width, but I'd stick with the 225/40/18 myself for either size wheel because that's already about a 25.08" diameter and a 1/2" wheel width increase won't really stretch it out that much.
You also want to be sure that the offset of the wheels is good for your car. I think the stock 16" offset is 53mm. I'm running 17" Rotas with a 48mm offset and you will need about the same offset on 18" wheels too!