I went through all this a few months ago when I got new rims...
Playing with a tire size calculator will help things make sense. Check out: http://www.1010tires.com/TireSizeCalculator.asp
Basically, larger rims do look better, but you will have to have a thinner tire on them to maintain the same diameter. If the diameter of the tire changes drastically, your speedometer will give you wrong numbers.
For example, if you went with the 225/40/18s your speedometer will read more than 4% slow.
To keep the diameter the same, you'd have to get the 35 profile tires. They'll look bad-ass, but will give you a very harsh ride. The thinner the profile, the less "cushion" the tires will provide.
Also keep in mind, the larger the rim, the heavier it is. Heavy rims will affect the car's performance since it takes more work from the engine to spin up all that rim.
Because of all this, I went with the lightest 17 inch rims I could find and a 45 profile tire. They still looked good, and the ride is comfortable.
Also check out the reviews section of this site, theres a bunch of stores listed there with good prices on rims and tires:https://www.celicahobby.com/reviews/showcat.php?cat=27