If you do decide not to upsize diameter wise, you might want to run aftermarket so that you can go with a wider wheel. This will allow wider tires in the future. Make sure to adjust the wheel offset accordingly to recenter the wheel in the wheel well. If the factory has the wheel location optimally centered and keeps the same scrub radius as stock. (Although I wouldn't be too concerned about effect of offset versus scrub radius unless it changes more than 1/2" from stock.) However usually they leave more clearance to the outside of the fenderwell than the inside.
Most of us with stock rims have updated to a 215/50. It's as wide as recommended for that width rim. This gives a slightly higher sidewall for rim protection. Plus the wider tire will distribute the impact load over a wider area.
Note:
I believe a 235 can fit with the right width/offset rim if the car is not lowered.
Going from the stock 205/50R17 and a 235/40R18 will reduce your sidewall by .75 inches.
If you went to a 235/45R17 instead, you would increase sidewall by about .12". But this tire is too wide for the stock rim.
Let's see what rim you would need for the 235 tire:
The max rim width for a 205/50 is 7.5" and the minimum allowed for a 235/45R17 is 7.5". So by changing to a 7.5" rim, you can keep your current tires, and run the wider ones after the originals wear out. You'll need to adjust the offset to fit though.
My factory rims are 17x6.5 with 52.5mm offset (dimensions cast into the rim). You can use this calculator to determine what the new offset for the wider rim would be with a 7.5" wheel:
http://www.gtsparkplugs.com/WheelOffsetCalc.html Don't forget to include the effect of a wider tire when checking clearance.
I think if you do a search there are others that have run a 235 wide tire on an aftermarket rim.