225/40/18 or 235/40 if you can is usually the go with an 18 x 8 (but im sure one of the other gc owners will be able to help out with tyre sizing + fitment).
That or a quick call to Wheels World and they'll you whether it can be done.
As for Ultrex wheels....DO IT (just did mine and am have trouble concentrating at Uni....if you get what I mean). Will look awesome on your gc.
Welcome Welcome
